Name: Custom Constant Force Springs
Brand: John Evans’ Sons
Availability: LimitedAvailability

Constant force springs are constructed of a tightly wound roll of strip spring material, which has extra-strong spring properties. The material is coiled to a specified diameter that produces constant torque when it is deflected. Once deflected, the material returns to its coiled diameter. Constant force springs offer long deflection without a significant change of force.

Understanding constant force spring dimensions—width and diameter—plays a crucial role in performance and application. As a balancing mechanism, the constant force spring is more compact and lighter than most other methods. It eliminates deadweights, linkages, cylinders, and other systems. As a return or retracting mechanism, it provides a smooth, even, uniform load through its entire length of travel (deflection). Constant tensioning or pressure is also an attribute.

Constant force springs are an excellent solution for reducing device costs and eliminating expensive, complex motors. At John Evans’ Sons, we are the leading international constant force spring manufacturer. We excel in designing and manufacturing springs for diverse applications, ensuring precision and reliability.

Many design factors will affect the time and cost needed to manufacture. Therefore, it’s important to work with a manufacturer like John Evans’ Sons that has the knowledge needed in the design phase. Our strongest selling points are our custom design capabilities, in-house tooling services, manufacturing, a wide and varied stock spring selection, and our commitment to quality and on-time delivery.

Leading Constant Force Springs Manufacturer

With over 175 years of spring manufacturing expertise, John Evans’ Sons delivers exceptional quality and precision-engineered solutions to clients worldwide. Our full range of turnkey spring and assembly capabilities is designed to meet the unique demands of your application. Our core offerings include:

Custom design services to tailor product solutions to your unique needs
In-house tooling services to deliver high-quality precision tools for your project
Advanced manufacturing services ranging from product design and prototyping to CNC machining, metal stamping/forming, heat treating, and laser etching
Quality assured products manufactured under ISO 9001:2015, ISO 13485:2016, and AS9100 quality management systems

We also maintain an extensive selection of standard springs in stock for immediate shipment as part of our commitment to on-time delivery.

Design and Performance Considerations

A constant force spring provides a smooth, uniform load through its full length of travel. This type of spring comes in compact package sizes, offers extensive linear range, and yields strong force within confined spaces. It also easily mounts onto existing hardware.

Multiple design factors have an impact on the lifespan of a constant force spring. To ensure that a spring will perform as intended in your specific application, carefully consider the following:

Consult an Engineer

Mounting

To mount a constant force spring, tightly wind it on a drum and attach the free end to the loading force. The diameter of the drum should be 10-20% larger than its inside diameter.

Linear Movement

A tightly wrapped spring provides a smooth range of motion and constant linear force throughout its length of travel, or deflection. For this reason, constant force springs can easily fit either full or partial extension configurations and applications.

Full Extension & Spring Load

Once the spring is extended to a length of 1.25x its diameter, it reaches its maximum load and can maintain this constant force regardless of how far it extends or retracts. The spring’s load only depends on the diameter of the spring and the width and thickness of the material.

Consult an Engineer

Custom Constant Force Spring Design

Our team understands the benefits of customizing the design of constant force springs to fulfill the exact requirements of your application rather than making do with an off-the-shelf solution. Our precision customization capabilities allow you the flexibility to choose the following in your springs’ designs:

Spring length
Helix direction
Inside and outside diameter
Material type, size, and shape
Ends and finishes
Tolerances
Materials
Mounting

You can optimize a spring’s performance under its expected operating conditions based on the customization options you choose. Material selection, for example, is an important factor for improving spring longevity and functionality.

John Evans’ Sons provides custom-engineered constant force spring solutions as a single-source design and manufacturing partner. Our in-house production supports varying order sizes and helps maintain efficient lead times.

If your spring is designed with thin materials and a large coil diameter, it will typically offer good longevity but achieve less force. Thicker materials with a smaller diameter, alternatively, deliver high force but may fail prematurely from wear. Choosing the right thickness and coil diameter for your application’s requirements while taking spatial constraints into account will impact your springs’ achievable force.

The type of material you plan to use for your springs will affect their durability, stability, and force output over time. This choice will also determine how well your springs will hold up under certain environmental conditions. For example, if springs are being installed in equipment that will experience extreme or fluctuating temperatures, their material and design must be capable of handling these thermal conditions. Also, in applications that will expose your constant force springs to chemical agents, it’s important to choose a material and an applicable coating that’s resistant to those particular substances, so the springs won’t degrade.

For projects in which safety and precision are paramount, like when strip edges will be exposed, ensuring smooth edges is important for the best performance and durability.

Springs in applications that involve rapid accelerations or high speeds must endure greater wear, vibration, dynamic force, and even heat concerns. Designing a spring to account for these issues will support better and more reliable performance in such operations.

We design constant force springs for optimal stability so that they can deliver consistent force and maintain their integrity when faced with mechanical stress or harsh environments.

Backbend is an important design consideration because it can cause your constant force springs to perform suboptimally. It leads to anything from noisy operations and excessive wear to misalignment, diminished load capacity, and force deviations. To combat this for lasting springs, using sufficient lubrication will help you avoid problematic wear and friction.

Your constant force springs must be made of the right base materials and coatings to perform well and last long in chemically aggressive applications.

Intended temperature range is a key consideration when selecting spring design and materials so that the spring will not degrade under normal operating conditions.

Depending on your application, applying lubrication to your constant force spring can improve its performance and increase its longevity by minimizing the effects of wear and friction.

Back-to-Back

Tandem

Laminar

Cavity

Shaft Fitting

Bearing or Conduit

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Materials

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Applications of Constant Force Springs

John Evans’ Sons’ constant force springs are in countless applications, including the Mars rovers Curiosity and Perseverance, to name a few. Constant force spring mechanisms apply constant force throughout their entire length, preventing them from uncoiling while in use. We design and manufacture standard and custom constant force springs for a broad range of applications, some of which include:

Surgical Devices

Aircraft Doors

Windows

Electric Motors

Fire Dampers

Gym Equipment

Applications

Surgical Devices

Applications

Aircraft Doors

Applications

Windows

Applications

Electric Motors

Applications

Fire Dampers

Applications

Gym Equipment

Benefits Of Constant Force Springs

View Blog

Comparison and Selection Charts

We’ve provided the following comparison and selection charts to help you determine the ideal spring for your use.

Material Thickness	2,500 cycles		10,000 cycles		20,000 cycles		75,000 cycles		150,000 cycles
	Coiled Dia.	Load (lbs.)	Coiled Dia.	Load (lbs.)	Coiled Dia.	Load (lbs.)	Coiled Dia.	Load (lbs.)	Coiled Dia.	Load (lbs.)
0.002	0.15	1.3	0.17	1	0.22	0.65	0.33	0.32	0.44	0.17
0.003	0.22	2	0.25	1.5	0.32	1	0.48	0.5	0.66	0.26
0.004	0.3	2.6	0.34	2	0.44	1.3	0.65	0.65	0.89	0.35
0.005	0.36	3.3	0.43	2.5	0.55	1.6	0.81	0.8	1.11	0.44
0.006	0.44	4	0.51	3	0.65	2	0.95	1	1.33	0.52
0.008	0.59	5.2	0.69	4	0.87	2.6	1.28	1.3	1.78	0.7
0.01	0.73	6.6	0.86	5	1.08	3.3	1.59	1.6	2.22	0.87
0.012	0.87	8	1.03	6	1.3	4	1.91	2	2.67	1.04
0.014	1.02	9.3	1.2	7	1.52	4.6	2.22	2.3	3.07	1.2
0.016	1.16	10.6	1.37	8	1.73	5.3	2.53	2.7	3.6	1.4
0.018	1.31	11.9	1.54	9	1.96	5.9	2.85	3	4	1.56
0.02	1.46	13.2	1.71	10	2.18	6.6	3.2	3.3	4.44	1.74
0.022	1.61	14.5	1.89	11	2.37	7.2	3.48	3.6	4.89	1.91
0.025	1.81	16.5	2.14	12.5	2.72	8.2	4	4.1	5.55	2.18
0.028	2.04	18.5	2.4	14	3.04	9.5	4.48	4.6	6.23	2.44
0.032	2.26	20.5	2.65	15.5	3.36	10.2	4.94	5.1	6.9	2.7

Line drawing of two constant force springs, illustrating minimum and maximum working deflection, drum diameter, and thickness. Detailed line drawing—depicts two constant force springs showing the minimum and maximum working deflection, drum diameter, and thickness for precise design considerations.

Selection Charts for the Ideal Constant Force Spring

Part Number	Load	Max Work Deflection	Band Length	Fatigue Life (cycles)	Width	Free I. D.	Thickness
A 3X50-SL04D16	0.23	12"	16"	20,000	0.25"	0.516	0.004
A 3X50-SK04D15	0.32	12"	15"	13,000	0.25"	0.438	0.004
A 3X50-SL05E17	0.36	12"	17"	20,000	0.3125"	0.656	0.005
A 3X50-SH03C10	0.37	8"	10"	2,500	0.1875"	0.219	0.003
A 3X50-SK04E15	0.4	12"	15"	13,000	0.3125"	0.438	0.004
A 3X50-SL05F30	0.43	25"	30"	20,000	0.375"	1.0	0.005
A 3X50-SH04C14	0.49	12"	14"	2,500	0.1875"	0.297	0.004
A 3X50-SK05E16	0.49	12"	16"	13,000	0.3125"	0.563	0.005
A 3X50-SL06F24	0.52	18"	24"	20,000	0.375"	0.781	0.006
A 3X50-SH04D14	0.66	12"	14"	2,500	0.25"	0.297	0.004
A 3X50-SL06G24	0.7	18"	24"	20,000	0.5"	0.781	0.006
A 3X50-SK06F23	0.71	18"	23"	13,000	0.375"	0.672	0.006
A 3X50-SH04E14	0.83	12"	14"	2,500	0.3125"	0.297	0.004
A 3X50-SL08G31	0.93	24"	31"	20,000	0.5"	1.047	0.008
A 3X50-SK06G23	0.95	18"	23"	13,000	0.5"	0.672	0.006
A 3X50-SH05E15	1.03	12"	15"	2,500	0.3125"	0.359	0.005
A 3X50-SK08G30	1.26	24"	30"	13,000	0.5"	0.875	0.008
A 3X50-SL10J33	1.46	24"	33"	20,000	0.625"	1.328	0.010
A 3X50-SH06F21	1.48	18"	21"	2,500	0.375"	0.438	0.006
A 3X50-SK08J30	1.58	24"	30"	13,000	0.625"	0.875	0.008
A 3X50-SH06G21	1.97	18"	21"	2,500	0.5"	0.438	0.006
A 3X50-SK10J32	1.98	24"	32"	13,000	0.625"	1.109	0.010
A 3X50-SL12K42	2.09	30"	42"	20,000	0.75"	1.578	0.012
A 3X50-SK10K32	2.37	24"	32"	13,000	0.75"	1.109	0.010
A 3X50-SH08G28	2.63	24"	28"	2,500	0.5"	0.578	0.008
A 3X50-SL12P42	2.8	30"	42"	20,000	1"	1.578	0.012
A 3X50-SK12K40	2.84	30"	40"	13,000	0.75"	1.344	0.012
A 3X50-SH08J28	3.29	24"	28"	2,500	0.625"	0.578	0.008
A 3X50-SL15P46	3.5	30"	46"	20,000	1"	1.984	0.015
A 3X50-SK12P40	3.79	30"	40"	13,000	1"	1.344	0.012
A 3X50-SH10J29	4.12	24"	29"	2,500	0.625"	0.734	0.010
A 3X50-SL15R46	4.37	30"	46"	20,000	1.25"	1.984	0.015
A 3X50-SK15P42	4.74	30"	42"	13,000	1.0"	1.672	0.015
A 3X50-SH10K29	4.95	24"	29"	2,500	0.75"	0.734	0.010
A 3X50-SK18P47	5.69	32"	47"	13,000	1.0"	2.0	0.018
A 3X50-SL20R54	5.83	36"	54"	20,000	1.25"	2.625	0.020
A 3X50-SH12K36	5.94	30"	36"	2,500	0.75"	0.875	0.012
A 3X50-SL20S54	6.99	36"	54"	20,000	1.5"	2.625	0.020
A 3X50-SH12P36	7.92	30"	36"	2,500	1.0"	0.875	0.012
A 3X50-SL25S59	8.74	36"	59"	20,000	1.5"	3.297	0.025
A 3X50-SK20S52	9.48	36"	52"	13,000	1.5"	2.219	0.020
A 3X50-SH16P38	10.6	30"	38"	2,500	1.0"	1.156	0.016
A 3X50-SL25U59	11.7	36"	59"	20,000	2.0"	3.297	0.025
A 3X50-SK25S57	11.9	36"	57"	13,000	1.5"	2.781	0.020
A 3X50-SL31U69	14.4	42"	69"	20,000	2.0"	4.078	0.031
A 3X50-SK25U57	15.8	36"	57"	13,000	2.0"	2.781	0.025
A 3X50-SH20R47	16.5	36"	47"	2,500	1.25"	1.469	0.020
A 3X50-SK31U68	19.6	42"	68"	13,000	2.0"	3.438	0.031
A 3X50-SH20S47	19.8	36"	47"	2,500	1.5"	1.469	0.020
A 3X50-SH25S48	24.8	36"	48"	2,500	1.5"	1.828	0.025
A 3X50-SH25U48	33	36"	48"	2,500	2.0"	1.828	0.025
A 3X50-SH31U58	40.9	42"	58"	2,500	2.0"	2.266	0.031

Available from our distributor, Stock Drive Products. Tel: 516-328-3300

Constant Force Spring Technical Information and Diameter vs. Load Chart (matl: T301 SS, calculations shown are at 1.0" width, interpolate for other spring material widths)

	2,500 cycles		10,000 cycles		20,000 cycles		75,000 cycles		150,000 cycles
Material Thickness	Coiled Dia.	Load	Coiled Dia.	Load	Coiled Dia.	Load	Coiled Dia.	Load	Coiled Dia.	Load
0.002	0.15	1.3	0.17	1.0	0.22	0.65	0.33	0.32	0.44	0.17
0.003	0.22	2.0	0.25	1.5	0.32	1.0	0.48	0.50	0.66	0.26
0.004	0.30	2.6	0.34	1.5	0.44	1.3	0.65	0.65	0.89	0.35
0.005	0.36	3.3	0.43	2.5	0.55	1.6	0.81	0.80	1.11	0.44
0.006	0.44	4.0	0.51	3.0	0.65	2.0	0.95	1.00	1.33	0.52
0.008	0.59	5.2	0.69	4.0	0.87	2.6	1.28	1.30	1.78	0.70
0.010	0.73	6.6	0.86	5.0	1.08	3.3	1.59	1.60	2.22	0.87
0.012	0.87	8.0	1.03	6.0	1.30	4.0	1.91	2.00	2.67	1.04
0.014	1.02	9.3	1.20	7.0	1.52	4.6	2.22	2.30	3.07	1.20
0.016	1.16	10.6	1.37	8.0	1.73	5.3	2.53	2.70	3.60	1.40
0.018	1.31	11.9	1.54	9.0	1.96	5.9	2.85	3.00	4.00	1.56
0.020	1.46	13.2	1.71	10.0	2.18	6.6	3.20	3.30	4.44	1.74
0.022	1.61	14.5	1.89	11.0	2.37	7.2	3.48	3.60	4.89	1.91
0.025	1.81	16.5	2.14	12.5	2.72	8.2	4.00	4.10	5.55	2.18
0.028	2.04	18.5	2.40	14.0	3.04	9.5	4.48	4.60	6.23	2.44
0.031	2.26	20.5	2.65	15.5	3.36	10.2	4.94	5.10	6.90	2.70

Quality Standards and Certifications

John Evans’ Sons is committed to high standards of quality, as evidenced by our ISO 9001:2015, ISO 13485:2016, and AS9100 certifications. Our team has quality assurance measures in place to help guarantee product precision and dependability while ensuring your satisfaction with your springs. Download our certifications to learn more or review our company’s quality policy.

Product Resources

Below, browse through our helpful collection of selection charts, sales sheets, and POP display information, all available for download as PDFs.

Icon representing a constant force spring selection chart, showing various options based on specifications such as force, size, and application. Choose wisely—use the constant force spring selection chart to find the perfect match based on force, size, and application needs for optimal performance.

Stock Constant Force Spring Selection Chart

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Icon representing a constant force spring balance spring sales sheet, detailing specifications, and available options for purchasing. Make informed choices—the constant force spring balance spring sales sheet provides key details on specifications and available options for your needs.

Constant Force Balance Springs Sales Sheet

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Pusher Variable Force Springs for POP Displays

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Window Balance Springs

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Window Balance Springs

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STOCK Constant Force Spring Selection Chart

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Industries

Our constant force springs are vital components in a wide variety of industries, such as:

Military

Aerospace

Robotics

Medical Devices

Large blue truck driving fast on the road with a truck behind blurred in the distance. The trucking industry depends on high-quality springs for optimal safety and efficiency.

Automotive, Trucking, & Transportation

Man touching the screen of point of purchase machine in self-checkout with blurred grocery store background. John Evans’ Sons, Inc. works with display companies, retailers, and merchandisers to optimize and secure their point-of-purchase displays.

Point of Purchase Displays

Inside look of electric motor. John Evans’ Sons is a leader in design and manufacturing of custom springs, metal stampings, motor brush springs, constant force springs and spiral torsion springs.

Electric Motors

Windows in manufacturing warehouse, stacked up and ready to ship. As the world leader in constant force spring technology,John Evans’ Sons (est. 1850) has produced well over 100 million high performance window balance products.

Window Manufacturing

Frequently Asked Questions

What’s the maximum load you can achieve with constant force?

A single spring can achieve a maximum of 60 lbs. Through options of using multiple springs, either back-to-back or in tandem or laminating the coils together higher loads can be achieved. Our ability to customize designs to suit your requirements allows us to work with you to achieve those requirements in a cost-efficient and timely manner.

What are some of the different applications?

John Evans’ Sons constant force springs are in countless applications, including the Mars rovers Curiosity and Perseverance. Constant force spring mechanisms apply constant force throughout their entire length, preventing them from uncoiling while in use. We design and manufacture standard and custom constant force springs for a broad range of applications, some of which include: counterbalances (windows, doors), surgical devices (clamps), stored energy (motor systems), brush springs (motors), and clamps.

What is the best constant force spring material?

The best constant force spring material varies depending on the type of environment it will be used in, such as corrosive and high-temperature environments, which can also affect the overall life of the spring and selected material.

Contact Us Today

With more than 175 years of experience in spring manufacturing, John Evans’ Sons is renowned for producing custom, high-performance constant force springs. Our precision-engineered solutions are tailored to meet the specific requirements of our clients’ applications, ensuring reliable performance.

Our seasoned professionals are available to assist you in achieving exceptional results that align with your project objectives. Contact us for more information on our constant force springs, or request a quote today to start your next project.

Request Your Custom Constant Force Spring Design

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Company

Industry

Application

Custom Constant Force Springs Manufacturer

Leading Constant Force Springs Manufacturer

Design and Performance Considerations

Mounting

Linear Movement

Full Extension & Spring Load

Custom Constant Force Spring Design

Constant Force Spring Mounting Configurations

Materials

Applications of Constant Force Springs

Benefits Of Constant Force Springs

Comparison and Selection Charts

Selection Charts for the Ideal Constant Force Spring

Selection Charts for the Ideal Constant Force Spring

Constant Force Spring Technical Information and Diameter vs. Load Chart (matl: T301 SS, calculations shown are at 1.0" width, interpolate for other spring material widths)

Quality Standards and Certifications

Product Resources

Stock Constant Force Spring Selection Chart

Constant Force Balance Springs Sales Sheet

Pusher Variable Force Springs for POP Displays

Window Balance Springs

Window Balance Springs

STOCK Constant Force Spring Selection Chart

Industries

Frequently Asked Questions

What’s the maximum load you can achieve with constant force?

What are some of the different applications?

What is the best constant force spring material?

Contact Us Today

Request Your Custom Constant Force Spring Design

Personal Information

Shape & Material

Measurements

Capacity Performance

Installation & Application