How Rings Work

 

Information provided in this section is strictly theoretical based on ideal conditions. Actual performance is heavily dependent on the physical properties of the mating components and their ability to withstand the forces generated by the Tolerance Ring.

Other factors affecting performance are the hardness, surface finish, lubrication and assembly procedure. A performance - testing program is strongly recommended in order to establish the final component diameters and tolerances within the range of the Tolerance Ring capability. For validation testing, production intent components are required.

Basic Principles of Tolerance Rings:

The Tolerance Ring is a precision-engineered device made form a thin spring steel strip of material into which waves, corrugations or bumps are formed. The strips are cut to length and curled into the ring shape. The waves are either facing inward or outward to accommodate different applications. The AN style, waves facing inward, is designed so that the ends can be closed and fitted into a bore or tube and is self-retaining when released. The BN style, waves facing outward, is designed so that when the ends are opened it will slip over a shaft and also be self-retaining when released.

The waveforms are designed to exert a holding force yet allow for ease of assembly between mating components. This closed wave configuration is typically very strong with the strongest part being the shoulders or closed ends of the wave. When the Tolerance Ring is assembled between mating components each wave is elastically deflected resulting in holding force. The holding ability of the ring is the resultant force of all the waves and the coefficient of frication with the mating components.

Fa (Retention Force) = CFr

C = Coefficient of friction
Fr= Spring Force Radially

Fr (Spring Force Radially)= KX

K = Spring Rate
X = Wave deflection

K (Spring Rate) = 4.8 x E x W x (T/P) 3 < lbs./in or N/mm>

E= Elastic Modulus for the ring material (KNmm2)
W= wave width (inch or mm) – not to be    confused with ring width
T = strip thickness (inch or mm)
P = pitch (inch or mm)

Within the elastic limit of the wave configuration simple spring theory applies. The major factors influencing spring rate are strip thickness and wave pitch. By varying these, a wide range of spring stiffness can be designed into a given component envelope. Typically a range of greater than two order of magnitude is achievable within the same space envelope.

Wave Compression

X = wave height – radial gap between components
X = wave height – (inner component OD – outer component ID)
                                      2

Component tolerance is a major factor in spring force. Wave height varies with each ring configuration within limits of material thickness and pitch. In general, the greater the wave height, the greater the elastic limit to handle greater tolerance and thermal differential.

Each wave exerts a spring force. Therefore, the greater number of waves, the higher the total force. Coupling this with the yield strength of the mating components and coefficient of friction allows us to estimate retention force, radial load capacity and torque capacity.

Things affecting our ability to accurately estimate the output for a specific application are:

1. Selecting a ring style and material, ring diameter and width and then specific wave configuration, fitting within the design constraints. First looking at available tools.
2. Evaluating mating components for hardness – ability to hold the force of the ring and surface finish – affecting the coefficient of friction.
3. Evaluating effects of operating temperature if dissimilar materials are involved. Also evaluate need for lubricants if movement between parts is required
4. Reviewing assembly process with design constraints.

As a result we recommend a testing program to verify the final design using production intent components.

 

From the assembly curve we see that the force increases as the mating components are pressed together. The force at the leading edge peaks and then drops off as the moving component slides over the wave then peaks again at the other end of the wave. Once completely over the wave the force drops off and stabilizes. The ring now conforms to the mating components and some slight plastic set has occurred. At this point the resultant spring rate can be determined.

Typical Compression Ranges in Different Applications:

Bearing Mount – 0-9% could go higher to accommodate greater thermal affects.

Torque Transfer – 8-16%

Torque over load – 14% - 20%

Axial Slip - 16-35% with non STD wave profile

Principles of operation in a bearing mounts & simple fixing: Applies to mounting tubes, solid shafts or pins especially where low assembly force is desired or if parts need to be disassembled in the field. It is not recommended that the ring be reused since it may have been damaged during use or when disassembling components.

* Refer to spring rate curve above where the ring is used primarily at lower end of the curve in the elastic range.

The most common application is retaining ball bearings.

The AN style rings are typically used to retain the outer bearing ring from spinning in the bore. It is preferred that the bearing have a slip fit to the shaft, thereby assuring that no load is applied through the bearing balls during assembly. However if the bearing is pressed onto the shaft and assembly requires pressing through the bearing a light duty ring is recommended with low spring rate. Centered or piloted mounting arrangements prevent overstressing the tolerance ring under high radial loads. In centered or piloted applications the rings are narrower than the bearing. Special ring configurations can reduce the number of active waves while reducing the assembly force and still giving good radial capacity.

The BN style rings are used to retain the inner bearings ring on to the shaft.

It is necessary to have a means of holding the ring in position on the shaft during assembly. This can be a groove on the shaft, a shoulder, a retainer ring or some other means built into the assembly tooling. In a centered arrangement the ring is narrower than the bearing and is used to assure concentricity and radial load capacity. During assembly the force is to be applied uniformly in one stroke and applied only to the inner-bearing ring.

Other bearing applications include sleeve bearings or bushings and certain needle bearings.

Tabbed rings available in limited sizes for critical applications with high vibration, vertical mounts or cyclic loads. Contact our engineering staff for additional information and recommendations.

Typical advantages:

1. Replace a slip fit allowing relaxation of tolerances, at times allowing for as cast or molded components, eliminating complex and costly machining.
2. Producing lower assembly force than a press fit, preventing bearing damage.
3. Improving mechanical coupling, matching harmonics with the mating component – replacing rubber boots.
4. Allowing the use of a less expensive bearing by controlling the internal clearance.
5. Allowing for angular misalignment, eliminating the need for special housing tooling and added housing length needed for spherical bearings.
6. Compensating for thermal expansion with dissimilar components
7. Offering high radial load capacity
8. Allowing axial movement
9. Reducing heat-transfer from outer components and allowing the bearing to run cooler, increasing bearing life.

Principles of operation in a torque drive application: Impellers, Fans, Cams, Gears & Pulleys. Applies when two components need to be rigidly mounted together so axial retention & torque transfers are assured.

* Refer to spring rate curve above where the ring is in both the elastic and plastic range.

Depending on the application and design constraints either AN or BN rings are used.

Depending on the torque, a higher deflection is typically required. This assures that the min torque is maintained over a greater range of mating component tolerance and thermal expansion. However, an evaluation of radial loads must accompany this to assure that external radial loads do not overstress the ring.

Typical advantages:

1. Assures minimum torque transfer.
2. Allows relaxation of tolerances, at times allowing for as cast or molded components.
3. Compensates for thermal expansion with dissimilar components
4. Allows the use of alternate materials to reduce cost or noise and vibration
5. Handles radial loads
6. Allows axial and radial adjustment – Zero top dead center timing is possible with no backlash.
7. Eliminates the need for complex and expensive machining required with splines, keyways or lock pins.
8. Eliminates need for threaded retainers and lock washers.

Principles of operation in a torque limiting application: For those situations where a torque overload would result in damage, would reduce shock loading or improve wear by allowing parts to slip and then operate in a new area.

*Refer to spring rate curve above where the ring is use primarily in the plastic range where wave compression range has the least effect on force/ torque.

These applications require critical control of all three of the mating components, the inner member, the outer member and the tolerance ring. An in-depth engineering evaluation and feasibility study are required. Most applications will also require a test and development program followed by extensive validation testing with production intent components.

Applications are limited by the number of slips and speed as well as other design criteria.

Depending on the application and design constraints either the AN or BN ring is used. These applications require a controlled torque range and therefore work in the flattest part of the force deflection curve where there is little change in force over a deflection range. Component tolerances are directly related to the torque range and must be controlled, typically within .001” (.0025mm). Use of dissimilar materials is also further limiting factor. The lower limit of torque is that torque required to assure normal operation with some safety factor. The upper limit of torque is that torque that would cause damage or excessive torque transfer (overload). Alternate ring designs with defined slip surfaces are used for these applications. These rings don’t use conventional or typical forming processes and will incur significant tool costs in most instances. For an application where slip is more that an occasional event, at speeds higher then hand turning, then lubrication is normally also required.

Typical advantages:

• Allows the use of lighter weight materials and less expensive components.
• Prevents damage to expensive components, resulting in equipment failure and down time.
• Controls slippage, allowing parts to move and eliminating high wear areas.
• Reduces or eliminates the effects of shock loads

Principles of operation in axial slip applications: Anywhere that two components need to slip axially relative to one another with a fixed range of force such as telescopic or energy absorption applications.

*Refer to spring rate curve above where the ring is used in full plastic deformation to handle very large tolerance ranges but have a fixed force after assembly. Force range is dependant on the tolerance range and wave configuration. Force varies by number of waves.

Applications in this category require engineering evaluation and a feasibility study. Development testing with production intent components is also necessary in most applications.

Depending on the application and design constraints either AN or BN rings are used. These applications require a controlled axial force range and work entirely in the plastic range of the waveform. Alternate wave configurations are used to allow for large plastic deformation without collapse. Rings in this category can also incur significant tool changes. Limited thermal expansion can be accommodated in these applications. Component tolerances, on the other hand, are loosely related to the force range and large commercial tolerances can be tolerated. Applications use DOM tubing without any secondary machining having typical tolerances of +/-0.0635 mm and 0.05 mm out of round. Development is under way with ERW tubing with typical tolerance of +/- 0.13 mm and 0.10 mm out of round. Surface finish must be relatively consistent over the slide distance.

Typical advantages:

• Large component tolerances
• Ease of assembly – Force at assembly is the controlled force to assure force is within range.
• Flanges can be added to hold rings in position
• Can accommodate for out- of- round components
• Large range of force available

a greater radial load capacity, torque capacity, or width than may be provided by a single Tolerance Ring, more than one ring may be used. It is necessary to separate the rings by a flange or shoulder to prevent the rings from overlapping.

 

 

 

 

USA TOLERANCE RINGS
85 Route 31 North, Pennington, New Jersey 08534
phone:609-745-5000; fax:609-745-5012