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Slewing Ring Bearings - Technical (Turntable Bearings)

Service Factors

To determine if a Silverthin™ Slewing Ring bearing is appropriate for an application, a SERVICE FACTOR is applied. Refer to the table below for a guide to the service factor to apply to your application. The load rating curves shown in this catalog are approximate, and represent an application service factor of 1.00. To determine the required bearing rating, multiply the applicable service factor by the applied loads on the bearing, and compare the resultant loads to the load rating curves.

Class of
Typical Considerations Application Examples Minimum
LIGHT Well defined loading Tire mounted light duty construction 1.00
Loading well below capacity Light duty Index table 1.00
Rotation slow, <10% of time and intermittent Light duty industrial manipulator or robot 1.00
Light duty hand operated mechanism 1.00
Light duty medical devices 1.00
Light duty aerial platforms 1.00
Welding positioners 1.00
Rotating signs, displays 1.00
MEDIUM Well defined loading Track mounted light duty construction 1.10
Loading near or below capacity Scrap yard construction 1.25
Rotation slow, <30% of time and intermittent Medium duty industrial manipulator or robot 1.25
Conveyors 1.10
Rotary tables 1.25
Capstans and turnstiles 1.10
Wastewater treatment 1.10
HEAVY Loading not well defined Forestry handling equipment 1.50
Loading beyond machine capacity can occur Heavy duty index tables and turntables 1.50
Shock loading can occur Excavators 1.50
Rotation intermittent, up to 100% of time
SPECIAL Loading not well defined Alternative energy (wind, hydro, etc) TBD
Continuous rotation Offshore application TBD
High speed rotation Amusement rides TBD
Heavy loads, shock, impact Steel mill applications TBD
High precision, positioning Precision robotics TBD

If you require any assistance in determining an applicable service factor, or would like a more detailed load rating curve (recommended if your service factor adjusted applied loads fall close to, or beyond, the load rating curves shown in this catalog), please contact Silverthin™ Engineering for assistance. Please note that the equipment designer is responsible for determining the correct service factor, often validated by testing.

Typical Application

“Typical application” of Silverthin™ Slewing Ring Bearings will exhibit the conditions listed below. Special consideration must be given to bearing selection and features whenever the application conditions differ from those considered “typical”. Those typical application conditions are:

  • Vertical axis of rotation. Essentially, the bearing mounted “flat”.
  • Compressive thrust and moment loads being predominant compared to tension loading.
  • Radial load limited to less than 10% of the thrust load.
  • For single row bearings, intermittent rotation (not continuous) should not exceed a pitch-line velocity of 500 feet/minute.
  • Operating temperature between -40ºF to +140ºF.
  • Mounting surface geometry and installation procedures to assure roundness and flatness of both races. An example approach would be to apply a centered thrust load while tightening the bolts using the alternating star pattern method.
  • Periodic checking of mounting bolts to verify proper tension is provided for.
  • Periodic lubrication is provided for.

Load Capability

Silverthin™ Slewing Ring Bearings are designed to accommodate significant radial, thrust and moment loads as shown below:

drawing showing different types of bearing loads

diagram of four point contact This is accomplished in most cases by the unique four point contact raceway geometry, which is similar in concept to Silverthin™ X-Type Thin Section bearings. This allows a single bearing to accommodate all three loading scenarios noted above, either individually or a combination thereof.

For more detailed information on SilverThin™ slewing ring load capabilities consult the load curves charts.


Silverthin™ Slewing Ring Bearings are used most commonly where rotation is slow, oscillating, and/or intermittent. For speed limit calculations please contact Silverthin Engineering.


Silverthin™ Slewing Ring Bearings are not typically provided with diameter tolerances. Some slewing ring applications require a higher degree of accuracy. For engineering and design support on special applications please contact Silverthin Engineering.


Silverthin™ Slewing Ring Bearings are often used indoors, and outdoors where exposure to moisture and significant contamination is possible. Normal temperature ranges -40°F to +140°F (-40°C to +60°C) are standard. Slewing rings designed to operate in harsher environments are available from Silverthin, contact a Silverthin Engineer early in your design process to identify the best bearing system solution for extreme environments.

Drawing explaining tension vs compression

Mounting – Tension versus Compression

As mentioned earlier, it is best to mount the bearings in “compression” as shown below. This ensures that the load is carried by the balls, which is represented in the load curve provided. Tension mounting has significantly less capacity, as then the bolt strength becomes the primary consideration for capacity.


Mounting surfaces need to be machined accurately for proper function of the bearing. Where standard bolt patterns cannot be accommodated, contact Silverthin Engineering for alternative options. Consideration must be given to mounting in tension or compression. In tension, BOLT strength becomes the limiting load consideration, the load curve no longer applies, and special considerations must be made. See additional guidelines below.

Minimum Mounting Structure Guidelines

Generally, this rule of thumb will provide adequate structural integrity.

Drawing showing minimum mounting structure guidelines

Flatness & Mounting Surface Dish (Mounting Surface)

Flatness of the bearing mounting surface is critical to optimal performance. Frequently mounting structures are welded or worked in a way to induce stresses into the structure. These stresses must be relieved, following which the bearing mounting surface must be machined flat. Flatness must be considered:

Drawing showing radial distance and dish
  • Circumferential Direction (δr): The amount of out-of-flatness allowable in the circumferential direction for four-point ball bearings is shown in the figure below. This amount of out-of-flatness must not be exceeded in a span less than 90°, and not more than once in a span not more than 180°.
  • Allowable Dish or Perpendicularity Deviation in the Radial direction (δp): For four-point contact ball bearing designs, this amount of dish allowable can be approximated using the formula:
    δp ≈ 0.001 ∗ Dw ∗ P
    P = radial dim of the mounting structure face (in)
    Dw = rolling element diameter (in)

Note that if an application requires greater precision or low rotational torque, it may be necessary to reduce the values of δr and δp. For roller bearings, the amount of flatness allowable is approximately 2/3 of that for an equivalent sized four-point contact ball bearing.

Chart: Allowable Deflection vs Raceway Diameter


Grease is the most common lubricant used in slewing ring bearings and gear applications. Regular lubrication through provided grease fittings or grease holes is required for proper operation on standard slewing rings. For special lubrication options, contact Silverthin.™

Friction Moment (Rotation Torque)

The Friction Moment can be estimated for a slewing ring bearing using the formula noted below. The resulting values assume that the bearing is mounted according to the guidelines outlined in this catalog. This estimate only applies when load is applied to the bearing, and does not reflect starting torque in an unloaded condition. Also not considered are frictional torque generated by the lubricant, seals and weight of the components. This does however provide a starting point, and with additional experience adjustments can be made in the assembly to accommodate for additional torque.

Mf = μ ∗ (4.4M + Fa Dpw + 2.2 Fr Dpw) / 2
Mf = Bearing starting torque under load (ft-lbs)
μ = Coefficient of friction (0.006 typically)
M = Moment load (ft-Ibs)
Fa = Axial load (Ibs)
Fr = Radial load (Ibs)
Dpw = Bearing pitch diameter (ft)


It is always suggested that bolts be selected with the advice and assistance of a fastening hardware supplier. Bolt quality, pretensioning procedures, and maintenance can vary widely.

The optimal bolting arrangement has a bolt circle in both the inner and outer races with equally spaced fasteners. This results in a more uniform mounting arrangement, yielding the best performance between the bearing and the fasteners. This is not always possible due to mounting structure arrangements, and holes may be shifted accordingly. In these cases testing is recommended to determine actual bolt loads, validate joint configuration and assembly procedure.

As a starting point to determine the approximate load on the heaviest loaded bolt, the following formula can be used. Please note that Silverthin™ makes no warranty, expressed or implied, regarding bolt adequacy. It is strongly recommended that testing be performed to determine the actual load, as this is the only reliable way to be certain.

RB = 12 ∗ M ∗ r ± Fa
BC ∗ n n
RB = Total load on heaviest loaded bolt (Ibs)
M = Moment load (ft-Ibs)
r = Rigidity factor. Use 3 for bearings and support structures of average stiffness.
Fa = Axial load (lbs)

If Fa is in tension, the sign is +

If Fa is in compression, the sign is -
Refer to section “Mounting - Tension versus Compression”

BC = Bolt circle diameter (in)
n = Total number of equally distributed bolts
Sf = Bolt factor of Safety. Minimum recommended value = 3. See formula below.
Sf = Bolt Proof Load Rating
Bolt Diameter (in) Proof Load (lbs)
1/2 17,000
5/8 27,100
3/4 40,100
7/8 55,400
1 72,700
1 - 1/8 91,600
1 - 1/4 116,300
1 - 1/2 168,600

Other Bolting Recommendations

  1. Use hexagon head high strength bolts with coarse threads according to SAE J429, Grade 8 or ASTM A490/A490M or ISO 898-1, Grade 10.9 tensioned to 70% of their yield strength.
  2. Use hexagon head coarse thread nuts where applicable according to SAE J995, Grade 8 or ASTM A563, Grade DH or ISO 898-2, Class 10.
  3. For optimal bolt tension, the ratio of the distance from the bottom of the bolt head to the first thread of engagement should be 3.5 or greater. Testing is required for validation.
  4. All mounting bolts in a given ring should have equal clamp length.
  5. The distance between the head of the bolt and the bolt threads should be at least equal to the bolt body diameter.
  6. Thread engagement length of the bolt in the mating steel structure should be at least 1.25 times the bolt diameter.
  7. Bench tests are recommended to validate that the bolt tensioning method achieves desired results prior to equipment testing.

Securing Bearing to the Mounting Surface

When installing the bearing, it is important to ensure that the bearing is as round as possible. This will optimize load distribution and promote the smoothest operation. The following procedures are recommended as an aid.

Diagrams showing star pattern bolt tightening sequence

Use hardened round flat steel washers in accordance with ASTM F436 under the head of the bolt, and also the nut. Lockwashers, and locking compounds on the thread, are not recommended.

Install the washers, nuts and bolts in the bearing and supporting structure and hand tighten. Do not distort the bearing in order to install bolts. Apply a moderate centered thrust load to the bearing. Tighten the bolts to the equipment designer’s specifications. A common approach is to use a star pattern to tighten the bolts, sequences as shown in the diagram below. The pattern is usually done in 3 steps at approximately 30%, 80% and 100% of the final bolt torque or tension level specified by the equipment designer.

Loss of proper tension can lead to premature bolt failure, failure of the bearing and structure, damage to components, and fatality or injury to anyone in the vicinity. The bolts require frequent inspection for proper tension, which is commonly accomplished by measuring torque of the bolt.

Silverthin Bearing Group of Mechatronics, Inc.
Toll Free 866.294.5841 • Fax 425.222.7535 • email
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