Bearing Installation & Retention
The Pin or Bolt | Housings | Corrosion Resistance | Installation | Spherical Bearing Installation
Lined Sleeve Bearing Installation | Retention Methods | Threaded Retainer Retention | Bolted Plate Retention
Housing Stake Retention | V-Groove Retention | Staking Procedure | Staking Force | V-Groove Staking Tool
Installation and retention details are important considerations when designing a bearing. Features such as pins or bolts, housings, corrosion resistance, installation method, and retention methods must be considered to ensure optimum bearing performance.
A typical bearing installation, which is staked into the housing, is assembled with a mating clevis, bolt, nut, washers, and plain and flanged bushings.
In most applications, the bolt is preloaded with the nut to clamp up the ball and force the ball to rotate on the race I.D. Caution must be exercised when clamping the ball. Excessive force expands the ball and will bind it in the race. If the ball is not clamped up, motion will usually take place on the bore, in which case the bolt, the bearing bore, or both must have suitable surfaces for this motion.
In addition to carrying the structural loads through the joint, the pin or bolt may function as a journal, and must therefore meet the multiple requirements of adequate strength, minimum wear, low friction, and corrosion resistance. In these instances, the following provisions for relubrication should be made:
- PTFE lines the bearing bore or the pin or bolt O.D.
- Dry film the bearing bore and/or the pin or bolt O.D.
- Introduce lubrication holes and grooves in the pin or bolt or the ball members
Suggested pin materials are 17-4PH and PH13-8Mo stainless steel, and 4130/4340 steel chrome plated .002 thick. Pins, either bare or plated, should be heat treated to the required shear strength (108,000 psi Ref.) and ground and polished to the required dimensions with a surface finish of 8 Ra or better. The recommended fit between the pin or bolt and the bearing bore is line-to-line to .001 loose.
Chamfered Size Calculation for
The housing into which the bearing is mounted must be designed to ensure the structural integrity of the bearing. The recommended housing dimensions are as follows:
- Bearing-to-housing fit: .0002 tight to .0008 loose.
- Bore finish: 32 Ra.
- Round within the bore diametral tolerance.
- Bore aligned perpendicular to housing faces within .002 for sleeve bearings only.
- Housing width: .005 tolerance (for staking purposes).
- For V-groove retention the housing bore is chamfered. Chamfer size is calculated as shown above.
- For housing stake and bolted plated retention, break edges .005 max on both sides.
The recommended shaft and housing sizes are based on an operating temperature range of -65° to 350°F. At elevated temperatures, allowances must be made for different coefficients of expansion for the various shaft, bearing, and housing materials. In general, the mating components should be adjusted to provide the recommended fit at operating temperature. In addition, internal bearing fit-up between the ball and race may be required (either additional internal clearance or decreased torque) to ensure proper operation over a broad temperature range.
The use of heavy interference fits between a bearing and housing is not generally recommended because it reduces internal clearance. If the application requires a heavy interference fit, the assembly of the bearing and housing must be accomplished by use of temperature differentials to prevent galling of the bearing or housing. The temperature differentials are dependent on the amount of press fit. After assembly, the bearing usually cannot be replaced because of galling during pushout. When using interference fits, the internal ball to race fit-up must allow for the contraction of the race (which can be up to 100% of the interference fit, depending on housing material, heat treatment, and size).
A bearing, housing, or shaft interface is a likely place for various forms of corrosion to develop. Corrosion may be initiated or accelerated by wear (fretting) or may be caused by the galvanic action of dissimilar metals in the presence of an electrolyte. Control of galvanic corrosion can be established by isolating and protecting the active metal surfaces. When corrosion resistant materials are used for bearings, pins or bolts and housings, there is little problem with galvanic corrosion. When dissimilar, noncorrosion resistant materials are used, precautions must be taken to protect bearings, shafts, and housings used in contact with other metals or with the atmosphere. The table below shows various bearing, shaft, and housing materials, with finishing precautions necessary to combine them to make a complete design. In addition to these recommendations, the bearing O.D. and housing bore are sometimes coated with zinc chromate primer according to TT-P-1757, epoxy primer according to MIL-PRF-23377, or sealant according to MIL-PRF-81733.
Treatments to Prevent Galvanic Corrosion
| Bearing Material
(Bore and O.D. Surface)
|Housing or Shaft Material|
|Aluminum Alloys||Low Alloy Steels||Titanium||Corrosion Resistant Steels||Super-Alloys|
|Bronze and brass||A,C||C||S||S||S|
|Bronze and brass cadmium plated||A||C||—||S||S|
|52100 and low alloy steels||A,C||C||—||C||C|
|440C stainless steel||A,C||C||S||S||S|
|440C with wet primer||A||C||S||S||S|
|Corrosion resistant steels, 300 series (17-4PH, 15-5PH, PH 13-8Mo, etc.)||A,C||C||S||S||S|
|Superalloys (Rene 41®, etc.)||A,C||C||S||S||S|
— = incompatible
A = Anodize aluminum per MIL-A-8625, Type II, or Alodine per MIL-C-5541
C = Cadmium plate per Fed-Spec QQ-P-416
S = Satisfactory for use with no surface treatment required
The installation of a bearing or sleeve into a housing bore is a simple operation when done properly. Alignment of the bearing or sleeve to the housing bore is critical to prevent a cocking motion during insertion, which can damage or ruin the bearing or housing. Temperature differential installation is recommended.
Use of an arbor press or hydraulic press is recommended. Under no circumstances should a hammer or any other type of shock-inducing impact method be used. A suitable installation tool (shown below) is advised. A guide pin aligns the ball in a 90° position, but all force is applied to the outer race face only. A lead chamfer or radius on either the bearing or housing is essential.
Spherical Bearing Installation
The same general procedure as outlined for spherical bearings should be followed (below). In the case of fabric lined bores, however, it is mandatory that both the insertion tool guide pin and the mating shaft have ends free of both burrs and sharp edges. A .030 (min.) blended radius or 15° lead is recommended, since it is virtually impossible to install a sharp edged shaft without inflicting some damage to the fabric liner. For maximum support of the fabric lined bore, the effective length of the insertion tool guide pin should exceed the sleeve bearing length.
Sleeve Bearing Installation
Bearing retention in a housing can be accomplished by any one of the methods listed in the table below. To determine the best method, several factors must be taken into account, such as effect on bearing internal clearance and torque, effect on housing residual stress, thermal expansion, added space and weight, retention capability, housing damage during bearing replacement, and number of times a bearing can be replaced.
The four retention methods listed in the table below are the most commonly used. Other methods do exist, such as adhesive bonding, snap rings, and threaded cover plates, but they should be used only as a last resort.
Characteristics of Recommended Retention Methods
|Method||Effect on Bearing Internal Clearance||Effect on Housing Residual Stress||Added Space and Wt.||Retention Capability Requirements||Can Replacement Damage Housing?||Possible No. of Replacements|
|Threaded Bearing Retainer||None||None||None||Medium||No||No limit|
|Bolted Retainer||None||None||High||High||No||No limit|
|V-Groove Stake||None||None||None||Medium||No||No limit|
|Housing Stake: Continuous or Interrupted||High||High||None||Low||Yes||None|
Threaded bearing retainers offer an excellent bearing retention method due to ease of bearing replacement, high axial thrust load capabilities, and ease of assembly in areas where accessibility to conventional staking would be difficult.
For high retention capability and ease of bearing replacement, the bolted plate method is recommended. However, space requirements and weight will increase.
Housing stake retention has many shortcomings when compared to V-groove staking. The major consideration is race contraction, which adversely affects internal fit-up. Housing stake retention should be used only when there is insufficient space on the race face for a V-groove or the race material is not ductile. When mounting, the bearing and its housing are supported by an anvil while the staking tool is forced into one side of the housing near the edge of the bearing. This action displaces a small amount of the housing material over the race chamfer. The opposite side of the housing is then staked in the same manner.
V-groove retention is the most widely used and recommended. The bearing outer race has a small groove machined into each face, which leaves a lip on the race O.D. corners. With the use of staking tools, these lips are swaged (flared) over the chamfered edges of the housing.The prerequisites for good V-groove staking are proper size housing chamfers, staking tools that match the V-groove size, and the availability of a hydraulic or pneumatic press capable of applying the staking force. To use V-groove staking successfully, the following conditions must be met:
- Race hardness: Rc40 max.
- Sufficient space on the race face for machining a groove. See V-groove sizes below.
- V-groove size capable of carrying the axial load.
- Install bearing into housing and position it symmetrical about housing centerline within .005.
- Mount bearing and top anvil over bottom anvil guide pin. (V-groove Staking Method)
- A trial assembly should be made for each new bearing lot to determine the staking force necessary to meet the axial retention load required. Excessive force should be avoided since this may result in bearing distortion and seriously impair bearing function and life. (See table for recommended Staking Force).
- Apply the staking force established by trial assembly, rotate assembly 90° and re-apply force.
- After staking, a slight gap may exist between race lip and housing chamfer as shown in the detail. This gap should not be a cause for rejection providing the bearing meets the thrust load specified.
Threaded Retainer Retention
Bolted Plate Retention
Housing Stake Retention
V-Groove Staking Method
*For PTFE lined bearings, add single liner thickness to "T Min."
The staking force equals the product of the bearing O.D. and a constant for each groove size (V-groove Size Table). For example, for a bearing with a “B” size V-groove and 1.500 O.D., the staking force will be 1.500 X 12,000 lbs. = 18,000 lbs.
These staking forces are valid for outer race materials having an ultimate tensile strength of 140,000 psi.
Staking forces for other materials are proportional to the ultimate tensile strength or the materials as compared to 140,000 psi.
These staking forces should be used as a general guide to establish a starting point. Lower forces may be adequate or higher forces may be necessary depending on staking technique and axial load requirements.
As a rule, only the amount of force required to get the desired amount of retention should be used.
The use of proper fits and staking techniques should not cause significant changes in bearing preload.
As a minimum, the first and last article staked should be proof-tested. A method for proof-testing staked bearings for axial retention is shown below. This is the generally accepted method for checking retention that bearing and airframe manufacturers use.
Staked Bearing Proof Testing Method
The graph below shows allowable design thrust loads for bearing O.D.’s The loads shown should be obtainable using staking tools with 45° outside angles.
Thrust Loads Based on Groove Types and Materials Specified.
The staking tool and staking anvil depicted below are made from tough, hardenable tool steel (for example, A-2) and heat treated to Rc55 to 60. The critical dimension of the tools are as listed. As a final check on the staking tool and anvil, a final layout drawing should be made to check fit-up. NHBB manufactures staking tools to meet many customers’ needs.
Staking Tool Design
Staking Anvil Design