Making and Installing Hammer Return Springs Guide

Clock movements with broken or missing hammer return springs reveal the critical problem where flat spring pressed into plate hole providing tension for hammer reset either falls out from insufficient friction fit or breaks from metal fatigue requiring fabrication and installation of replacement using spring steel material. When clockmakers encounter strike hammers that don't return to rest position after lifting or discover loose spring moving in mounting hole eventually falling out during operation, the challenging repair situation occurs because commercial replacement springs are rarely available for specific movement models requiring custom fabrication from suitable spring steel stock cut to proper dimensions and tempered for adequate springiness without brittleness. This demanding repair task happens because hammer return springs are simple pressed-fit components consisting of flat spring steel with narrow mounting leg inserted into drilled hole in back plate and wider spring portion projecting perpendicular to plate surface contacting hammer arbor pin providing constant tension pulling hammer downward to rest position between strikes. This guide covers complete hammer return spring fabrication and installation from selecting appropriate spring steel material to proper mounting techniques. You'll learn sourcing spring steel from salvaged mainsprings, feeler gauge blades, or tape measure springs avoiding need for expensive heat treatment, cutting springs using thin cut-off wheels in rotary tools creating accurate profiles matching original dimensions, forming proper mounting leg geometry where slightly wider base creates interference fit preventing spring from loosening in hole, installing springs through careful alignment and controlled hammering seating spring firmly without cracking, and understanding correct spring orientation where flat spring lies parallel to plate face contacting pin protruding from hammer arbor base rather than touching arbor shaft directly. The key to successful hammer return spring replacement is recognizing that spring must be cut from properly tempered spring steel rather than annealed steel requiring heat treatment while mounting leg dimensions are critical where excessive width cracks plate during installation but insufficient width allows spring to work loose and fall out during operation requiring balance between secure friction fit and safe installation forces.

Understanding Hammer Return Spring Function

Spring Purpose and Operation

Hammer return spring maintains hammer in rest position between strikes. After chime or strike train lifts hammer allowing it to fall striking gong or bell, spring pulls hammer back down to rest against stop. Without return spring, hammer remains elevated or falls randomly creating erratic striking. Proper spring tension ensures reliable hammer reset while allowing free lifting during normal operation.

Spring consists of two distinct sections. Narrow leg section inserts into hole drilled in back plate. This leg must be wide enough creating friction fit preventing spring from falling out but not so wide that installation cracks plate or requires excessive force. Spring section projects perpendicular from plate mounting point. This portion contacts pin on hammer arbor providing tension.

Spring contacts specific point on hammer assembly. Pin protrudes from hammer arbor base between hammer connection and back plate. Spring lies parallel to plate face resting against this pin not touching arbor shaft directly. Pin provides positive contact point ensuring consistent spring pressure throughout hammer rotation. Incorrect spring orientation contacting shaft creates variable tension as hammer moves affecting strike reliability.

Common Spring Failures

Springs fail through two primary mechanisms. Metal fatigue from decades of flexing causes spring to crack and break. Crack typically initiates at stress concentration point where mounting leg transitions to spring section. Progressive flexing propagates crack until spring separates completely. Broken spring no longer provides tension allowing hammer to remain elevated or fall unpredictably.

Mounting leg loosens from insufficient friction fit. If leg wasn't properly sized during original installation or if plate hole has enlarged from wear, spring works loose during operation. Loose spring moves in hole gradually working outward. Eventually spring falls completely out leaving hammer without return tension. Attempting to tighten loose spring by flattening mounting leg often causes breakage from work hardening previously flexed material.

Environmental factors accelerate failure. Rust or corrosion weakens spring material. Rust develops at mounting leg where moisture accumulates in plate hole or where spring contacts other components. Weakened material breaks easily during normal flexing. Similarly, excessive flexing from improper installation or incorrect spring dimensions causes premature fatigue failure. Spring designed for light tension breaks when forced to provide excessive return force.

Gilbert Movement Spring Configuration

Gilbert movements typically use flat spring mounted on back plate contacting pin on hammer arbor base. Spring orientation is critical for proper operation. Spring lies parallel to plate face not perpendicular. This horizontal orientation allows spring to push against pin providing consistent downward force on hammer assembly regardless of hammer position during rotation.

Pin location determines required spring length. Pin protrudes from hammer arbor base at specific distance from plate. Spring must reach from mounting hole to pin location providing adequate contact pressure. Too-short spring doesn't reach pin. Too-long spring requires excessive bending creating high stress and premature failure. Measure pin distance from proposed mounting hole location before cutting spring.

Some Gilbert movements have additional helper springs wound around upper posts assisting hammer return. These coil springs supplement main return spring providing additional tension for reliable operation. However, main flat return spring is primary component. Helper springs alone cannot provide adequate return force. Both spring types must function properly for reliable strike operation throughout movement life.

Selecting Spring Steel Material

Salvaged Mainspring Sources

Broken mainsprings provide ideal material for hammer return springs. Mainsprings are properly tempered spring steel requiring no heat treatment. Material thickness typically matches return spring requirements. Width is easily cut to needed dimensions using cutting tools. Most clockmakers accumulate broken mainsprings during normal repair work providing ready material source without additional cost.

Select mainspring section without cracks or severe kinks. Examine material under magnification verifying smooth surfaces free from rust or damage. Avoid sections near original spring breaks where metal may be weakened from fatigue. Use middle sections of long mainsprings where material experienced minimal stress during original operation. This ensures maximum remaining service life in new application.

Mainspring material arrives pre-tempered to proper hardness. No heat treatment is necessary. Simply cut to required shape and install. This simplicity makes salvaged mainsprings preferred material source when available. However, clockmakers without mainspring inventory must seek alternative materials. Commercial spring steel requires heat treatment or alternative pre-tempered sources must be found.

Feeler Gauge Blade Alternative

Feeler gauge blades provide excellent alternative spring steel source. Blades are manufactured from properly tempered spring steel ensuring adequate springiness without brittleness. Multiple thickness options allow selecting blade matching original spring thickness. Individual blades can be purchased inexpensively without acquiring lifetime supply of material. Small gauge sets provide variety of thicknesses for different applications.

Select blade thickness matching original spring. Most hammer return springs use material between 0.010 and 0.020 inches thick. Feeler gauge sets typically include blades in this range. Thicker blades provide stronger spring force. Thinner blades provide lighter tension. Match original thickness when possible. If original is unavailable, start with moderate thickness like 0.015 inches and adjust if spring force proves inadequate or excessive.

Feeler gauge material is clean and ready to use. No preparation is needed beyond cutting to shape. However, feeler gauge blades are narrow limiting available width for cutting. This restricts spring designs to relatively compact configurations. For wide springs exceeding feeler gauge blade width, alternative material sources are necessary. However, most hammer return springs fit within feeler gauge dimensions making this practical solution.

Tape Measure Spring Option

Retractable tape measure springs provide another readily available spring steel source. Tape material is tempered spring steel with appropriate flexibility for return spring applications. Tape is wider than feeler gauge blades allowing larger springs. Used or broken tape measures are often discarded providing free material source. However, tape measures have curved cross-section requiring flattening before use.

Extract spring from tape measure carefully. Unwind tape completely removing from housing. Cut desired length using aviation snips or similar cutting tool. Flatten tape using smooth-faced hammer and steel block. Work carefully avoiding kinks or sharp bends. Progressive gentle hammering gradually flattens curved tape into flat spring stock suitable for return spring fabrication.

However, tape measure material has limitations. Flattening process may alter spring temper creating unpredictable spring characteristics. Material may be too thick or too thin for specific applications. Width may exceed requirements necessitating lengthwise cutting creating additional work. Despite limitations, tape measure springs work adequately for many applications when other materials are unavailable. Test spring force after installation adjusting if necessary.

Cutting and Shaping Spring

Pattern Creation From Original

Use broken original spring as cutting pattern when available. Lay spring on selected material marking outline with scriber or fine permanent marker. Include all features - mounting leg, spring section, and any bends or offsets. Accurate pattern transfer ensures replacement matches original dimensions providing proper fit and function. However, if original spring is severely damaged or missing, estimate dimensions from movement geometry.

Measure critical dimensions when original is unavailable. Determine mounting hole diameter and location. Measure distance from hole to hammer arbor pin contact point. Calculate required spring length providing adequate reach with modest tension. Allow slight excess length for adjustment during installation. Mark these dimensions on material creating pattern for cutting. Test-fit rough-cut spring before final shaping verifying dimensions are appropriate.

Consider stress concentration points during pattern creation. Avoid sharp internal corners or abrupt transitions creating stress risers where cracks initiate. Use gentle curves transitioning between mounting leg and spring section. Radius corners reducing stress concentration. These design refinements extend spring life preventing premature fatigue failure. Small improvements in geometry dramatically affect long-term reliability.

Cutting Technique Using Rotary Tool

Thin cut-off wheel in rotary tool provides best cutting method for spring steel. Use wheels approximately 0.020 inches thick for precision cutting without excessive material removal. Secure material firmly preventing vibration during cutting. Work slowly allowing wheel to cut without forcing. Excessive pressure breaks wheels and creates rough edges requiring additional finishing work.

Make relief cuts before attempting complex curves. For inside corners or tight curves, drill small holes at curve centers. Cut straight lines from edge to holes. Remove waste material creating access for cutting actual curve. This technique prevents wheel binding in tight spots reducing breakage and improving cut quality. Use progressive passes for thick material. Multiple shallow cuts produce cleaner results than single deep cut.

However, rotary tool cutting has safety considerations. Wear eye protection preventing injury from wheel fragments or material chips. Secure work firmly preventing movement that catches wheel causing violent material ejection. Work in well-ventilated area avoiding dust inhalation. Thin cut-off wheels are fragile breaking unpredictably creating sharp flying debris. Careful technique and appropriate safety equipment prevent injuries during cutting operations.

Forming Mounting Leg

Mounting leg requires careful dimensional control. Leg must be narrower than mounting hole for insertion but wider for secure friction fit preventing spring from falling out. Typical approach creates leg slightly tapered. Narrowest section at tip allows easy initial insertion. Wider section near spring body creates interference fit when fully seated. This geometry provides progressive tightening during installation seating spring securely.

However, excessive leg width cracks plates during installation. Brass plates are particularly vulnerable. Measure hole diameter accurately. Cut leg width to approximately 0.005 to 0.010 inches wider than hole diameter. This modest interference creates adequate friction without excessive installation force. Test-fit leg in hole before final installation. Leg should enter partway with hand pressure requiring light hammering for complete seating.

Leg length affects spring positioning. Longer leg seats spring deeper in hole moving spring body closer to plate. Shorter leg leaves spring body farther from plate. Calculate proper leg length positioning spring body at correct distance for proper pin contact. Too-long leg over-seats spring creating excessive tension or preventing pin contact. Too-short leg leaves spring loose allowing excessive movement or inadequate tension. Measure carefully ensuring spring positions correctly when fully seated.

Installation Procedures

Hole Preparation

Clean mounting hole thoroughly before installation. Remove any debris, old spring fragments, or corrosion. Use appropriate size drill bit as cleaning tool. Rotate by hand clearing hole without enlarging dimensions. Compressed air removes loose particles. Clean hole ensures proper spring seating and maximum friction contact preventing future loosening. Damaged or enlarged holes may require repair before reliable spring installation.

Verify hole depth is adequate for mounting leg length. Drill blind holes to sufficient depth accepting entire leg without bottoming. Leg must seat fully drawing spring body to proper position. Insufficient hole depth prevents complete seating leaving spring loose. Test depth using drill bit or wire gauge matching leg length. If depth is inadequate, carefully deepen hole using appropriate size drill bit maintaining perpendicular alignment.

For severely damaged or oversized holes, bushing may be necessary. Press-fit brass bushing into damaged hole. Drill bushing to proper dimension for spring leg. However, bushing installation requires specialized tools and skills. For valuable movements, professional bushing is worthwhile. For common movements, relocating mounting hole slightly may be simpler solution. New hole location must maintain proper spring geometry for pin contact.

Spring Alignment and Seating

Align spring carefully before installation. Spring must be perpendicular to plate face preventing binding during insertion. Mounting leg must align precisely with hole. Misalignment causes leg to bend during installation creating weak point prone to failure. Use magnification observing alignment from multiple angles. Correct positioning before applying installation force prevents damage and ensures reliable seating.

Start spring insertion with hand pressure. Press firmly seating leg partway into hole. Spring should enter smoothly without excessive force. If significant resistance is encountered, verify alignment and check leg dimensions. Don't force misaligned or oversized spring. Forcing creates plate damage or spring deformation compromising installation. Remove spring, correct problem, and restart installation with proper alignment.

Complete seating using light controlled hammer blows. Use small punch or drift supporting spring during hammering. Work carefully avoiding spring deflection or plate damage. Progressive gentle taps seat spring gradually. Observe spring movement during hammering ensuring perpendicular advancement without tilting. Stop when spring body reaches proper position for pin contact. Over-seating draws spring too close to plate creating incorrect geometry.

Final Positioning Verification

After installation verify spring contacts pin properly. Rotate hammer arbor observing spring-to-pin relationship. Spring should maintain contact throughout rotation providing consistent tension. Gap between spring and pin indicates misalignment or incorrect dimensions. Contact on arbor shaft rather than pin creates variable tension affecting operation. Correct positioning is essential for reliable hammer return throughout service life.

Test spring tension adequacy. Manually lift hammer releasing slowly. Spring should pull hammer firmly downward without excessive force. Weak spring allows hammer to remain elevated or return slowly. Excessive tension restricts hammer lifting or accelerates spring fatigue. Proper tension balances reliable return against longevity. If tension is inadequate, thicker material or different spring geometry may be necessary. Excessive tension requires thinner material or reduced spring length.

Verify spring doesn't interfere with other components. Check clearance to adjacent mechanisms during complete operating cycle. Spring shouldn't contact other levers, pivots, or gear teeth. Interference creates friction, noise, or operational problems. Slight spring bending may improve clearances if interference is minimal. However, significant interference requires spring removal and modification preventing long-term problems from inadequate clearances.

Troubleshooting and Adjustments

Inadequate Spring Tension

Weak spring tension manifests as hammer failing to return to rest position. After striking, hammer remains partially elevated or returns slowly. This creates erratic striking as subsequent lifts begin from incorrect starting position. Inadequate tension results from too-thin material, incorrect spring geometry, or insufficient contact with pin. Diagnosis requires systematic evaluation of spring characteristics.

If material thickness is inadequate, replace spring using thicker stock. Test various thicknesses determining minimum providing reliable return. However, excessively thick material creates high stress concentrations increasing fatigue failure risk. Balance adequate immediate tension against long-term durability. Proper material selection provides reliable operation throughout expected service interval.

Incorrect spring geometry also causes weak tension. If spring is too short, contact pressure against pin is minimal. Lengthen spring or reposition mounting hole closer to pin increasing spring deflection at rest position. This increases contact force pulling hammer downward more firmly. However, excessive deflection causes premature fatigue. Optimize spring length and mounting position balancing tension requirements against stress levels ensuring adequate performance with acceptable longevity.

Excessive Spring Tension

Overly strong spring creates different problems. Hammer doesn't lift freely during normal operation. Strike train labors against excessive spring resistance potentially stopping if power is marginal. Hammer may not lift fully reducing strike force creating weak tone. Strong spring also accelerates fatigue failure from excessive stress during flexing. Proper tension provides adequate return without restricting operation.

Reduce tension by using thinner spring material. Replace spring with thinner stock providing lighter force. Test operation verifying hammer lifts freely while returning reliably. Find minimum thickness providing acceptable performance. Thinner material reduces stress improving longevity while maintaining necessary function. However, excessively thin springs may vibrate or resonate creating noise or erratic operation.

Alternatively adjust spring geometry reducing deflection at rest position. Shorten spring length or move mounting hole farther from pin contact point. Reduced deflection decreases contact force lightening spring tension. This approach retains original material thickness potentially improving fatigue life compared to using thinner material. However, geometric changes affect spring positioning requiring careful calculation ensuring proper contact maintenance throughout operating range.

Spring Loosening Prevention

Properly installed spring should remain secure throughout normal service life. However, some installations develop loosening from inadequate friction fit or vibration. If spring shows movement in mounting hole, reinstallation with improved retention is necessary. Several approaches increase holding power preventing future problems. Don't ignore loosening spring. Eventually it will fall out requiring emergency repair.

Simplest approach increases mounting leg width creating tighter interference fit. However, this risks plate cracking during installation. Alternative approach adds small amount of appropriate adhesive to mounting leg before installation. Cyanoacrylate or similar adhesive supplements friction fit preventing movement while allowing future removal if necessary. Apply minimal adhesive avoiding excess that interferes with seating or creates messy appearance.

For severely damaged mounting holes preventing reliable friction fit, create new mounting hole at slightly different location. Calculate position maintaining proper spring geometry for pin contact. Drill new hole ensuring adequate surrounding material. Install spring in new location filling old hole with brass plug or leaving empty depending on visibility and preference. This permanent solution provides reliable mounting when original hole is unusable.

FAQs

What material should I use to make hammer return springs?

Use properly tempered spring steel from salvaged mainsprings, feeler gauge blades, or tape measure springs avoiding need for heat treatment where broken mainsprings provide ideal material being pre-tempered spring steel requiring no heat treatment with thickness typically matching return spring requirements. Most clockmakers accumulate broken mainsprings during normal repair work providing ready material source without additional cost. Feeler gauge blades provide excellent alternative manufactured from properly tempered spring steel with multiple thickness options typically including blades between 0.010 and 0.020 inches thick matching most hammer return spring requirements. Individual blades can be purchased inexpensively without acquiring lifetime supply. Retractable tape measure springs provide another readily available source though curved cross-section requires flattening before use. Avoid annealed steel like 1095 steel sheets from industrial suppliers unless you have heat treatment capabilities because annealed material lacks springiness requiring heating to red temperature, quenching in water or oil, and tempering to proper hardness - process requiring specialized knowledge and equipment. Pre-tempered spring steel from salvaged sources eliminates heat treatment complexity ensuring reliable results without specialized metallurgical skills.

How do I cut spring steel without breaking cutting wheels?

Cut spring steel using thin cut-off wheel approximately 0.020 inches thick in rotary tool working slowly allowing wheel to cut without forcing where excessive pressure breaks wheels and creates rough edges requiring additional finishing. Secure material firmly preventing vibration during cutting and make relief cuts before attempting complex curves by drilling small holes at curve centers cutting straight lines from edge to holes removing waste material creating access for cutting actual curve. This technique prevents wheel binding in tight spots reducing breakage and improving cut quality. Use progressive passes for thick material where multiple shallow cuts produce cleaner results than single deep cut. However rotary tool cutting requires safety precautions wearing eye protection preventing injury from wheel fragments or material chips and securing work firmly preventing movement that catches wheel causing violent material ejection. Thin cut-off wheels are fragile breaking unpredictably creating sharp flying debris making careful technique and appropriate safety equipment essential. Alternative cutting methods include jewelers saw for intricate work or metal shears for straight cuts though shears work-harden material at cut edges potentially creating crack initiation points requiring edge finishing before installation.

How wide should the mounting leg be for secure installation?

Mounting leg should be approximately 0.005 to 0.010 inches wider than mounting hole diameter creating modest interference fit providing adequate friction without excessive installation force risking plate cracking particularly with brass plates. Measure hole diameter accurately before cutting leg where leg must be narrower than hole for insertion but wider for secure friction fit preventing spring from falling out. Create slightly tapered leg where narrowest section at tip allows easy initial insertion while wider section near spring body creates interference fit when fully seated providing progressive tightening during installation. Test-fit leg in hole before final installation where leg should enter partway with hand pressure requiring light hammering for complete seating. Excessive leg width cracks plates during installation while insufficient width allows spring to work loose and fall out during operation requiring balance between secure friction fit and safe installation forces. For severely worn or damaged holes preventing reliable friction fit add small amount of cyanoacrylate adhesive to mounting leg before installation supplementing friction fit or create new mounting hole at slightly different location calculating position maintaining proper spring geometry for pin contact.

Where exactly should the spring contact the hammer assembly?

Spring should contact pin protruding from hammer arbor base between hammer connection and back plate with spring lying parallel to plate face rather than touching arbor shaft directly because pin provides positive contact point ensuring consistent spring pressure throughout hammer rotation. Spring orientation is critical where spring must lie horizontally along plate face not standing perpendicular to plate. This horizontal orientation allows spring to push against pin providing consistent downward force on hammer assembly regardless of hammer position during rotation. Incorrect spring orientation contacting arbor shaft creates variable tension as hammer moves affecting strike reliability because shaft diameter varies and rotation changes contact point creating inconsistent spring force. Verify spring contacts pin properly after installation by rotating hammer arbor observing spring-to-pin relationship where spring should maintain contact throughout rotation providing consistent tension. Gap between spring and pin indicates misalignment or incorrect dimensions while contact on arbor shaft rather than pin creates variable tension. Some Gilbert movements have additional helper springs wound around upper posts supplementing main return spring but main flat return spring mounted on back plate contacting pin is primary component responsible for reliable hammer return.

Can I use annealed steel and heat treat it myself?

Yes you can use annealed 1095 steel from industrial suppliers and heat treat it yourself if you have metallurgical knowledge and appropriate equipment but process requires heating steel to red temperature approximately 1500 degrees Fahrenheit, quenching in water or oil for hardening, then tempering by reheating to lower temperature around 400-500 degrees creating desired spring characteristics. However heat treatment process has significant challenges including achieving uniform heating throughout material, maintaining proper temperature without pyrometer making temperature determination difficult, preventing warping or distortion during quenching, and controlling tempering temperature achieving springiness without brittleness. Improper heat treatment creates springs that are too soft lacking adequate return force or too brittle cracking during installation or normal operation. Most clockmakers without heat treatment experience prefer using pre-tempered materials like salvaged mainsprings or feeler gauge blades eliminating heat treatment complexity and ensuring reliable results. If you attempt heat treatment practice on scrap material first developing technique before making actual replacement spring and test spring characteristics after treatment verifying adequate springiness by bending slightly observing whether spring returns to original shape indicating proper temper or remains bent indicating insufficient hardness or cracks indicating excessive hardness and brittleness.

What if the spring keeps falling out after installation?

Spring falling out after installation indicates inadequate friction fit from undersized mounting leg or enlarged mounting hole requiring remedial action for reliable retention. Simplest solution increases mounting leg width creating tighter interference fit but this risks plate cracking during installation particularly with brass plates. Alternative approach adds small amount of appropriate adhesive like cyanoacrylate to mounting leg before installation supplementing friction fit preventing movement while allowing future removal if necessary where you apply minimal adhesive avoiding excess that interferes with seating or creates messy appearance. For severely damaged mounting holes preventing reliable friction fit despite proper leg sizing create new mounting hole at slightly different location calculating position maintaining proper spring geometry for pin contact. Drill new hole ensuring adequate surrounding material providing structural integrity then install spring in new location filling old hole with brass plug or leaving empty depending on visibility and preference. This permanent solution provides reliable mounting when original hole is unusable from wear, damage, or previous repair attempts. Don't attempt repeatedly reinstalling spring with same dimensions as this work-hardens mounting leg eventually causing breakage. Address root cause through proper leg sizing, adhesive supplementation, or hole relocation ensuring secure long-term installation.

How do I know if spring tension is correct?

Correct spring tension allows hammer to lift freely during normal operation while providing firm positive return to rest position after striking where you test by manually lifting hammer then releasing slowly observing return behavior. Proper tension pulls hammer downward decisively without excessive force. Weak spring allows hammer to remain elevated or return sluggishly creating erratic striking as subsequent lifts begin from incorrect starting position. Excessive tension prevents hammer from lifting freely during normal operation where strike train labors against spring resistance potentially stopping if power is marginal and hammer may not lift fully reducing strike force creating weak tone. Balance adequate return force against operational freedom where spring should provide minimum force necessary for reliable return without restricting lifting. If tension is inadequate after installation replace spring using thicker material or increase spring deflection by lengthening spring or repositioning mounting hole closer to pin contact point. If tension is excessive replace with thinner material or reduce deflection by shortening spring or moving mounting hole farther from pin. Test adjustments systematically making small changes and evaluating results before additional modifications ensuring optimal balance between reliable return operation and long-term durability throughout expected service interval.