Slewing ring technical

The full QCB Catalogue which contains this information is available on the DOWNLOADS page

Slewing Rings - General

Slewing rings usually consist of an inner ring and an outer ring which are separated by ball or roller rolling elements.

The rings may incorporate a gear and tapped or clearance holes are provided for bolts. The rolling elements are usually separated by spacers and the raceways usually hardened and ground. Nitrile rubber strip seals retain lubricant in the bearing, and prevent the ingress of contaminants. Grease fittings allow re-lubrication.

Many variants to the above general description exist in practice.

When compared to traditional methods of providing rotation, power transmission and fixation, slewing rings are seen as an elegant and cost effective solution.

Slewing Ring Loads

Slewing rings can accommodate axial, radial and moment loads of any magnitude and in combination to provide either oscillatory movement or full rotation.

The loads and duty cycle of the machine must be properly defined so that an adequately sized, yet economical slewing ring can be selected - simply designing to maximum load is wasteful.

Although it is possible to analyse the raceway and bolt load curves graphically, for greater accuracy QCB uses modern software to study the effect of these loads in terms of raceway safety, bolt safety and gearing performance.

Proper analysis of the gearing can require detailed analysis for which QCB has a range of software tools at its disposal.

Customers are invited to download a copy of the Technical Enquiry form and submit it to QCB for assistance, especially if there is legislation covering the design such as DNV, Lloyds, ABS requirements for marine or offshore applications.

Transport, Storage & Mounting Preparation

As supplied, slewing rings are packed for storage in a dry (humidity < 75%), covered and vibration free store for a period not exceeding 12 months.

Slewing rings should only be transported and stored horizontally or on specially built incline racks as they can deform in long term storage under their own weight. Shock loads should be avoided to limit any damage to the raceways. Care must be taken not to dislodge seals or grease fittings during movement. Wooden or rubber spacers should be used to separate unwrapped bearings stacked vertically to avoid surface damage. Gear teeth should be protected from impact damage.

Light surface corrosion may occur in humid conditions but is easily removed from the external surfaces - it is more important that the raceways are well greased to remain rust free. For this reason QCB bearings are usually 100% grease filled at source. (This may vary for historical or customer related reasons). Many QCB bearings also have a black phosphate coating which not only enhances their aesthetics, but also provides a small degree of corrosion resistance.

In cases of extreme long storage the bearings may need professional dismantling, cleaning and regreasing before use. For this reason QCB bearings that are sold after long term storage are stripped and inspected before release.

Either the standard threaded bolt holes or the lifting bolts provided should be used to lift the bearings in a flat plane. For safety check the weight of the ring to ensure any slings or chains used are suitable. Slewing rings should not be suspended from a crane by a single sling or placed on the ground vertically as they may deform under their own weight.

Degrease and clean the bearings with standard commercial solvents (without chlorine) - especially the mounting faces. Use sparingly and do not let this fluid work its way under the seals into the raceway. Extra grease is the most effective barrier to contamination


QCB uses 2 basic steels for the majority of its production program.

Code "CC" - 50Mn or C45 Normalised
Code "AA" - 42CrMo4 Quenched & Tempered

All QCB geared Slewing Ring bearings as well as any QCB Slewing Rings > 1000mm outside diameter are made in AA Steel as standard.

Aluminium or Stainless steels are used for special applications.

Rolling elements are generally manufactured in 100Cr6 but Ceramic ball have been used in some high speed applications. Spacers are usually Nylon - but brass or even steel strip cages are used in special designs.

The Standard QCB seal strip is a Nitrile rubber, VITON seals are available on request. Other seal arrangements can be used if needed.

Support structures

Although slewing rings are designed to support large axial, radial and moment loads they are inherently elastic structures that must be supported by flat, machined companion surfaces which are rigid enough to eliminate torsional buckling under load that would affect the smooth operation of the ring.

The width of the supporting flanges must at least equal the width of the ring it supports. Thick circular supports are preferred over thinner supports with reinforcement ribs.

The use of modern FEA analysis may allow the use of thinner flanges.

Where there are high radial forces the ring should be properly supported on the reference diameters to ensure the bolts are not overloaded in shear. The suggest thickness of the supporting rings is tabulated below.

Raceway Ø mm500750100012501500200025003000
Minimum support thickness mm2530354050607080

Flatness defects

Under load, flatness defects must not exceed the values tabulated to avoid tight spots or seizure, both of which will reduce the operating life of the ring. The values tabulated are for "long wave deviations" around the circumference. Note that X-roller bearings demand finer tolerances than ball bearings for smooth operation.

Shorter wave defects (e.g. between 2 bolt holes) must not exceed 1/4 of this value.

Defects in the radial direction (referred to as "conicity") mus not exceed 0.05mm/m based on the raceway diameter.

If the tolerances above cannot be met by normal machining methods then an epoxy resin such as "Chockfast" may be used.

Raceway Ø mm500750100012501500200025003000
Ball bearing0.
X-Roller bearing0.

Slewing ring orientation

Slewing rings are assembled by hand. Balls or rollers and spacing elements are inserted one by one through the filling plug(s) which are usually situated in the soft zone left during the raceway hardening process.

Filling plugs are held in place by a tapered pin and a subtle modification in the raceway profile limits the load carrying capacity in that zone. As a result, the filling plugs should be positioned at the point of minimum stress (usually at right angles to the main load axis).

In some cases an asymmetric bolt pattern or dowel pins will force a specific alignment.

In OEM bearings intentional non-symmetry may be useful to line up greasing points with automatic lubrication systems.


While QCB Light series slewing rings can be used with Grade 8.8 bolts, our larger, solid section slewing rings are generally designed to be used with Grade 10.9 hexagon head bolts or Grade 12.9 cap screws. QCB slewing ring bolt curves are calculated in accordance with standard practices (based on VDI 2230) and the grade of bolt is noted on the load curve.

THERE IS A COMMON MISCONCEPTION THAT A 14mm HOLE IN A SLEWING RING TAKES A 14mm BOLT. THIS IS NOT TRUE. Bolt holes in a slewing ring are generally designed to be clearance holes. In the case above some manufacturers specify a 13.5mm through hole to force choice of an M12 bolt. Special execution close tolerance bolt hole designs can be supplied to order.

If there are high radial loads, or if the slewing ring is to be used with a horizontal axis of rotation, the circularity of the slewing ring may be affected or the shear stresses generated in the bolts may be too high for safety. In these cases the bearing should be located in the structure using the toleranced location diameters provided as a standard feature on many QCB slewing rings.

Dowel pins can also be an effective way to increase radial capacity of the fixings for bearings that do not have machined location diameters..

For 50Mn steel (code "CC") the use of flat hardened washers is recommended to reduce surface pressures under the bolt head and nut. If cap screws are used, the specific pressure under the bolt head should not exceed the limits of the material. The use of any elastic spring or serrated washers is prohibited and their use will negate the warranty.

Untreated bolts should be lightly oiled and tightened progressively in steps of 60%, 80% and 100% bolt torque using a calibrated torque wrench, moving around the periphery of the slewing ring in approximately 120° steps. If the slewing ring is rotated during the tightening process any tight spots will become evident and should prompt immediate investigation.

Bolt torques should be checked before the machine enters service as a degree of "settling" may occur. Check again after 100 hours of operation and thereafter at least on an annual basis.

Recommended bolt torque figures are tabulated below. For large diameter bolts a hydraulic tensioning system should be used (indicated by * in table below).

Strength Class DIN/ISO 898Grade 8.8Grade 10.9Grade 12.9
Yield point Rp 0.2<=M16 640 Mpa>M16 660 Mpa940 MPa1100 Mpa
ISO Thread Stress x-section areaCore x-section areaTension forceTightening torqueTension forceTightening torqueTension forceTightening torque

Strength Class ANSI B1.1
SAE Grade 8 CoarseTensile x-section areaTightening torque
1/4-20 UNC0.03187.2
5/16-18 UNC0.052415
3/8-16 UNC0.077525.9
7/16-14 UNC0.106340.9
1/2-13 UNC0.141978
5/8-12 UNC0.226154
3/4-10 UNC0.334272
7/8-9 UNC0.462436
1-8 UNC0.606653
1.1/8-7 UNC0.763927
1.1/4 UNC0.9691299


QCB geared, metric slewing rings have a 20 deg involute spur gear machined into either ring (sometimes both!), but helical and worm gears bearings are also produced. Imperial dimension slewing rings use either a Fellowes stub, an American stub or American full depth gear.

All QCB geared slewing rings are manufactured in 42CrMo4 V steel for superior gear strength when compared to other economy brands.

Slewing rings gears often have an addendum correction to improve gear strength and mesh characteristics, and if so the pinion gear should be suitably corrected. QCB uses the most up to date calculation software available to offer an optimal solution for high-torque or multi-drive applications. The theoretical tangential gear strength figures (Fz-normal and Fz-max) are listed on most drawings.

A comprehensive range of metric and imperial pilot bore pinions is stocked (UK only) to suit the Light Series bearings.

The gear backlash must be set at the point of maximum eccentricity of the slewing ring, which is indicated by 3 painted teeth. Ensure each pinion is properly aligned with good contact over the full face width.

A backlash of between 0.03 - 0.05 x module is recommended for most applications. This distance is set by moving the pinion in a radial direction to the bearing axis, or by turning the drive motor on an eccentric flange mount.

During installation the slewing ring should be rotated a few times in each direction to check the gear mesh.

Wear on the gear flanks of up to 0.1 x module is permissible.

Fixed center systems must be accommodated at the design stage - i.e backlash can be designed into the gearing geometry.


QCB slewing rings are usually fitted with a nitrile rubber lip seal which rides on the surface of the adjacent ring. In many designs this can be exchanged for VITON seal strip.

Different seal arrangements for specific applications such as double seal systems, V-ring seals, quad section seals, metal shields and labyrinth seals with grease injection can be provided if required.

Seals should be inspected during annual maintenance and replaced if required. QCB UK stocks replacement seal strip for most bearings.

Speed limits

The speed of a slewing ring can be defined by the factor - i.e. the average diameter (mm) x speed (rpm)

QCB slewing rings are designed to work within the following limits:

Standard ball/spacer construction < 40 000 - 60 000
Ball / cage construction < 70 000 - 130 000
Crossed roller construction < 24 000 - 35 000

These figures can be increased for special applications - speak to our experts!


Raceway lubrication

QCB stock bearings are factory filled with a standard lithium base mineral oil EP2 grease. Regreasing is mandatory after long storage. Any good quality lithium based grease with a base oil viscosity of at least 200 c/s at 40C will offer adequate protection in most applications.

The regreasing interval is best determined by the environment

EnvironmentRecommended interval
Dry, clean workshop300 hrs duty or 6 months
Outside, exposed150 hrs duty or 4 monthly
Aggressive50 hrs duty or 2 monthly

The amount of grease required can be estimated by the following formulae:-

Single row ball bearing Grease qty (gr) = 0.7 x raceway Ø (mm) x ball ز (mm)/ 1000
Single row roller bearing Grease qty (gr) = 0.5 x raceway Ø (mm) x roller ز (mm) / 1000

During regreasing it is advisable to rotate the bearing to ensure even distribution of the lubricant. This will also avoid overpressurising the seal which can be forced out of its groove. Most slewing rings operate at high loads and slow speed and can be filled 100% full - this ensure the grease acts as a barrier to contaminants.

The onus remains with the client to ensure the bearing is adequately and freshly lubricated at time of installation.

Recommended lubricants are tabulated below. Other lubricants can be supplied against special order. (e.g. bio-degradeable greases)

BrandGreaseOperating temp range
BPEnergrease LS EP2-20 to + 120 C
ESSOBeacon EP2-20 to +120 C
TOTALMultis EP2-30 to + 120 C
SHELLAlvania EP2-25 to +120 C
FUCHSStabyl LT 150-50 to +120 C

Gear lubrication

Any gear should be lubricated immediately after fitting and setting of the backlash.

Suitable open gear grease should be sprayed or brushed on to cover the teeth completely. open gear greases will typically have a base oil viscosity of > 500c/s at 40C; good adhesive properties and a high resistance to washout.

Some lubricant manufacturers recommend a "running in grease", changing to an "operation grease" after time to allow the gears to settle in.

Some recommended greases are tabulated below.

BrandGreaseOperating temp range
BPEnergol WRL-20 to + 120 C
TOTALCarter open gear-20 to + 125 C
SHELLAeroshell 14-54 to +93 C
EXXONMOBILMobiltac 81-20 to +120 C

Operational clearance and wear

Properly specified and maintained, QCB slewing rings will offer years of satisfactory service.

After assembly the clearance or total deflection under load under known circumstances should be recorded to serve as a baseline for future maintenance. 

In service, slewing rings will usually wear at a linear rate, once this rate accelerates this is a sign that the bearing is reaching the end of its service life.  As a “rule of thumb”, once the measure deflection exceeds 1.5x the original value then plans should be set into motion to procure and replace the ring. If the measured deflection exceeds 2x the initial value the ring should be replaced immediately for safety reasons.

Customers should bear in mind that some large slewing rings may be long lead time items (6 -9 months). In some cases slewing rings can be reconditioned to bridge this gap.   

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