Tolerance rings: eliminating tight assembly tolerancesKen Thayer | May 24, 2019
Bearings are precision mechanical components that are manufactured to tolerances measured by ten-thousandths of an inch. Shafts and housings must be made to the same tolerance levels to ensure a proper fit. If the housing tolerances are too tight the bearing may be damaged during assembly. If the tolerances are too loose the bearing may move or vibrate during operation.
The American Bearing Manufacturers Association (ABMA) developed bearing tolerance standards for bearing producers in the early 1900s. In addition to bearings, the standards also cover shaft and housing dimensions. Tolerances vary depending on bearing size and series, shaft and housing rotation and load conditions.
It is important to machine shafts and housings on equipment with the precision to produce these design tolerances and to inspect them afterward.
The tables below illustrate the tolerances for an ABEC 1 medium series bearing with a 25 mm bore where either of the following conditions is true:
- Rotating shaft, stationary housing and stationary load
- Stationary shaft, rotating housing and rotating load
In Table 1 the shaft diameter specification is designed to create a slight interference fit with the bearing bore as the tolerances guarantee that the shaft diameter will always be greater than the bearing bore. This ensures that the bearing inner ring will not slip on the shaft while under load.
For the given conditions above, Table 2 shows that the housing specifications call for a slightly loose fit of the bearing outside diameter to the housing. In this case, the largest bearing outside diameter and the smallest housing bore would create a line-to-line fit.
Dimensions are also available for other bearing sizes, series, tolerance classes and load conditions. Manufacturers publish tables for all of these variables to help design engineers.
Machining shafts and housings to these tolerances adds to the cost of manufacturing. As a general rule, the tighter the tolerances, the more expensive the part is to make. What if there was an alternative, more cost-effective option?
The tolerance ring solution
Tolerance rings offer an alternative to precise machining tolerances. Tolerance rings are precision mechanical fastening components manufactured out of spring/carbon steel and alloys or stainless strip. They are used to join mating cylindrical components, such as bearings, gears, sprockets and pulleys. Tolerance rings can be used for axial retention, torque transfer and radial loading between mating parts.
Instead of the .0004” shaft tolerances indicated in Table 1, shaft or housing tolerances can be increased to as much as .002” or larger on larger diameters. This represents a significant reduction in complexity and production costs.
Tolerance rings are also a perfect solution for a damaged or worn shaft or housing and for bores that are accidentally machined oversized.
Tolerance rings are manufactured with a series of waveforms that act as radial springs when installed to create an interference fit between mating components. When assembled the waveforms flatten against the housing or shaft, securing the components. The tolerance ring fastens the components together without requiring tools, adhesives or other fastening components. Component disassembly is also simple and can be performed without risk of damaging the mating parts.
The basic operating principles of tolerance rings are the physics properties of springs and friction.
Spring formula: F = Kx where
- F = force to compress the spring (lbs or N)
- K = spring rate (lbs/in or N/m)
- x = distance the spring is compressed (in or m)
Friction formula: F1 =cN where
- F1 = friction force opposing movement (lbs or N)
- c = coefficient of friction
- N = force perpendicular to the surface (lbs or N)
Different tolerance rings are designed to be mounted onto a shaft or into a housing. If the ring is to be mounted to a shaft the waveforms point radially outward and the free-form diameter is smaller than the shaft diameter. When mounted to the shaft the ring conforms to and becomes self-retaining on the shaft. If the ring is to be mounted into a housing the waveforms point radially inwards and the ring conforms to the bore of the housing that contains it.
Tolerance ring mounting can be classified into three main types.
- Centered – Tolerance rings are assembled into a groove cut into the shaft or housing. The grooves capture and locate the tolerance ring axially on both sides, simplifying assembly. This creates improved part alignments, improved concentricity and allows for accommodation of excess radial or shock loads.
- Piloted – Piloted, or half-centered, mounting is similar to centered, but with lower manufacturing costs. Piloted shafts or housings feature one shoulder that can be used for axial location of the tolerance ring.
- Free – Tolerance ring is assembled in a thru-bore housing or straight shaft. This mounting does not provide axial support to the tolerance ring in either direction and subjects the tolerance ring to all radial loading. The ring must be axially supported during assembly of the mating parts.
Benefits of tolerance rings
Tolerance rings offer many benefits, including:
- Wider radial tolerance requirements
- Improved vibration characteristics
- Secure mounting across a wide temperature range
- Lower assembly time and costs
- Elimination of pins, keys, adhesives, threads and splines
- Compensation for small misalignments and removal of backlash
- Infinite rotational positioning of components prior to assembly
- Torque overload protection for products such as gears
An additional benefit of tolerance rings is that they can be used in lightweighting applications, a growing trend for automotive and other industries. Manufacturers have been replacing heavy steel housings with aluminum and other lighter materials. One of the consequences is that these materials exhibit different rates of thermal expansion, which can cause problems. If a steel bearing is mounted in an aluminum housing, the housing bore will grow due to thermal expansion at a greater rate than the outside diameter of the bearing. Tolerance rings work within this elastic range and will maintain a press fit of the bearing.
Tolerance rings mount bearings are found in many applications to assemble a variety of components.
- ABS toner rings
- Steering column anti-theft devices
- Timing sprocket camshaft connection
- Alternator bearings
- Blower motor bearings
- Electric motors
- Bearings in housings
- Bearings on shafts
- Phenolic brush holder
- Rotor tachometers on shafts
- Hard disk drives
- Power tools
- Ceramic bearings and sleeves
- Lawn and garden
- Rototiller know mounts
- String trimmer bearing mounts
- Material handling
- Ball transfer units
- Torque transfer applications
- Slip clutch on shaft or bore
Whether it is for an existing assembly to counteract a damaged housing or in a new assembly to secure a steel bearing to an aluminum housing, tolerance rings offer a unique, simple, inexpensive solution for a myriad of applications.