What is a wave spring
In motor bearing system arrangements, springs are often used to apply preload to some bearings. For large motors, column springs are generally used. For smaller motors, wave springs, or butterfly springs, are most often used.
Wave springs, as the name implies, are spring components that have a wavy shape. Usually, they are used in applications where the load and deformation are small, the spring stiffness is small, and the applied axial force is small. Their light weight and small size make them suitable for weight reduction and workplaces where space is limited.
In motor bearing systems, it serves to apply axial loads to the bearings to minimize bearing noise, as well as to avoid pseudo Brinell indentation.
- Reduced working height
In space-constrained applications, they save up to 50% space compared to traditional round wire springs. Space savings can be realized through the use of flat material and sinusoidal waveforms. As a result, fewer material requirements are needed for production, the spring is more compact, and the total weight of the spring and components can be reduced.
- Design flexibility
Based on a number of unique processes, any aspect of the spring can be specifically tailored to meet the requirements of the application. The number of coils, thickness, type of ends, waveform distribution and type of material can all be customized to the user’s needs.
- Save costs
It allows for a lower working height compared to traditional round wire springs. Allows the entire assembly to be reduced in size, requiring less material to produce, making manufacturing more economical and faster. For applications with outer perimeter castings or machined parts, significant savings in wave spring costs can be realized.
- Uniform strength distribution
Common applications that require an even distribution of force are soft materials such as plastics used in seals, valves and components. Unlike round wire springs, it allows for a more even distribution of force. During compression, round wire springs may bend or deform unexpectedly. The reduced height of products reduces similar errors. Force distribution is a key application issue, and it can be designed to have a flat end shape at one or both ends. With 360 degrees of contact, flat ends distribute the spring force more evenly across adjacent components.
It can be used in a variety of industries and are suitable for almost all applications. Example:
Flow valves: Countertop springs precisely control the linear displacement of the piston as fluid pressure increases.
Pressure Relief Valves: Air pressure under the assembly causes an increase in the spring load, forcing the plate away from the sealing surface, thus providing a mechanism for pressure reduction. Once the pressure is reduced, the spring returns to its original working height, allowing the unit to seal again.
Sealing Surfaces: They apply pressure to accurately load the mating surfaces, properly sealing the fluid.
Vibration Isolators: Under constant load, isolators dampen vibration during operation of the device. They are used to provide an accurate and predictable load/deflection curve.
Wave Spring Standards
The products are mainly used to apply an axial force in a shaft system. The deformation of this spring is the elastic force generated by axial compression. The resulting elastic force is actually the spring’s internal resistance to bending deformation.
The specific parameters can be found in JB/T7590 “Technical Conditions of Steel Wave Springs for Electric Machines” or obtained from suppliers of wave springs.
The material of the products is spring steel, the grade is 65Mn, after surface oxidation treatment, its hardness is between 45-52HRC.
The size range of them in the national standard is D16-D240, and their inner diameters range from 11.7 mm to 204 mm. Sizes exceeding this range require additional consideration or the use of alternative methods.
Elasticity of wave springs
It is easy to see from the standard that wave springs have a free height H. At the same time, in the standard test there is a test height h. That is to say, when the spring is deformed from the free height H to the test height h, the spring force should be within the range given by the standard.
If the deformation of the spring exceeds this range in actual working conditions, the spring force is not within this range. This is a common problem encountered by many motor manufacturers. Sometimes the spring deformation is too large, sometimes too small. The so-called too large or too small is not reached the test height in the standard.
On the other hand, the post-test stress and toughness of the products are specified in the standard. The size of the spring’s elasticity shall not be less than 90% of the minimum value after the test in the above standard. Similarly after a certain amount of bending, no damage should occur.
Therefore, simply applying pressure to the spring should not result in breakage or a huge change in elasticity as long as it is within the range of deformation.
Preloading of wave springs and motor bearings
Since the function of the wave spring in the motor bearing system is to apply axial and load, the spring force of this spring should be equal to the preload value that needs to be applied. That is, the spring force at the test height (i.e., the height after deformation) should be within the range of the axial preload value required for the bearing.
Wave spring applications
- Aerospace electrical connectors
The two single-layer hitch products in this device exert a constant force on the joint when compressed, providing the assurance of a continuous connection.
- Flow Valve Applications
As fluid pressure increases, the spring precisely controls the linear displacement of the plunger, which will position the fluid orifice to allow proper fluid flow.
- Multi-tooth cutting tools
The cutting tool uses a specially designed product that is manufactured with a locating tip. The wave spring applies pressure to the two halves of the cutting tool, binding them together while allowing them to oscillate.
When compressed air is applied through the valve, the wave spring compresses to accurately maintain a certain pressure that will precisely regulate the flow from the valve.
- Pressure relief switching valve
Exact pressure is achieved using this spring. Air pressure under this device causes the spring load to increase, forcing the plate away from the sealing surface, thus providing a pressure relief mechanism. When the load applied to the spring is reduced, the unit is ready to seal again.
- water valve
Wave springs, also known as flat wire compression springs, prevent the valve handle from rotating by maintaining a constant load and engaging the threads on the spindle. As the handle rotates counterclockwise, the spring’s resistance increases, reducing the likelihood of continued rotation.
- face seal
They apply pressure to properly seal liquids by accurately applying load to carbon steel surfaces based on mating surfaces. The products operate over a fixed operating range and provide exact force. It replaces stamped wave washers that do not maintain the necessary elasticity ratio. Exact pressure must be applied from the carbon steel surface to the sealing surface provided by the spring to avoid excessive wear while maintaining a proper seal.
Problems often encountered when applying preloads with bobbins
- Wave spring wear
Some engineers think that the spring surfaces are heat-treated and therefore wear will occur on the bearing surfaces. Is this really the case? The hardness of the outer ring of deep groove ball bearings is around 56 HRC, while the hardness of wave springs is 45-52 HRC as mentioned earlier, so when wear occurs, the springs will be more severe.
However, such wear is detrimental to the wave spring and therefore to the preload of the bearing. So why does such wear occur after the spring has been compressed and deformed? Theoretically, the outer ring of the bearing should not be rotating either, and even if there is a slight creep, such severe wear should not occur. (With the exception of the outer ring running)
This seems highly likely that the bearing system is vibrating during operation, and the direction of this vibration may have radial and axial components. As a result, contact friction between the bearing end face and the spring has occurred and such wear has occurred.
Of course there is also a possibility that the outer ring of the bearing is seriously running, but this factor can be easily seen by observing the outer ring.
- Wave spring breakage
We know from the standards for the products that wave springs have a certain degree of toughness. If an axial force is applied to a spring in a motor, the deformation of the spring within a reasonable range does not damage the ability of the spring. Even if the deformation range is exceeded and the spring loses its elastic capacity, it will not break considering the elasticity of the metal.
Fractures in metals are initially caused by fatigue, for the same reason as fatigue in bearings. Often, shear stresses are repeated a certain number of times, leading to fatigue.
There are two factors involved, one is the shear stress and the other is the recurrence. If the wave form spring only in a shear stress, it is difficult to fatigue fracture. That must be the result of the reciprocal occurrence of shear stress. For motors, the greatest possibility is that the motor is working in a vibration situation, and the spring keeps on compressing, rebounding, compressing again, and rebounding again.