Clearance vs. Preload in Slewing Bearings
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Clearance vs. Preload in Slewing Bearings: How to Choose for a Machine Tool Rotary Table
What each setting does, when a rotary table needs it, and what to put in your inquiry
Guide from Anhui Yuanfeng Slewing Rings Co., Ltd.
Quick Answer
Clearance is the small free movement inside a bearing; preload, also called zero clearance, removes that movement and puts the rolling elements under slight pressure. For rotary tables on boring mills, milling machines and vertical lathes, a preloaded or zero-clearance bearing usually gives better rigidity and positioning, at the cost of higher rotating torque. For slow, light-duty or non-precision applications, normal clearance is often enough and costs less. The right choice depends on your cutting load, accuracy target and drive power.
Key Takeaways
1. Clearance is normal, not a defect: a small amount of axial and radial play lets the bearing turn freely, absorbs thermal expansion and makes assembly possible.
2. Preload is a deliberate trade: removing the play raises rigidity, positioning repeatability and resistance to chatter, and raises rotating torque and heat at the same time.
3. Precision rotary tables usually need preload or zero clearance: any free play lets the table tilt or shift under cutting force, which shows up as poor surface finish and positioning errors after indexing.
4. More preload is not always better: too much raises torque demand, builds heat at speed, speeds up wear and makes the bearing less tolerant of uneven mounting surfaces.
5. Specify the condition in numbers: state the clearance or preload target, the direction to measure it, and your allowable torque, because "zero clearance" and "preload" are not the same thing to every supplier.
What Is Clearance in a Slewing Bearing?
Every slewing bearing has some freedom of movement between its inner and outer ring. This is measured in two directions:
- Axial clearance: how far one ring can move up and down relative to the other.
- Radial clearance: how far one ring can move sideways relative to the other.
Clearance is not a defect. A small amount allows the bearing to turn freely, tolerates thermal expansion, and makes assembly possible. Problems start when there is too much of it for the job, or none where some is needed.
What Is Preload (Zero Clearance)?
A preloaded bearing is built so that the rolling elements, balls or rollers, are slightly compressed between the raceways. There is no free play, and the bearing already carries a small internal load before any external load is applied.
Zero clearance is the common name for a bearing with no measurable play. Strictly speaking, a bearing can range from slightly loose, to just zero play, to a defined preload. Make sure you and your supplier use the same words for the same condition, and specify the target in numbers.
How They Compare
| Property | Normal clearance | Preloaded / zero clearance |
|---|---|---|
| Free play | Small but present | None |
| Rigidity under load | Lower | Higher |
| Positioning repeatability | Fair | Better |
| Resistance to vibration and chatter | Lower | Higher |
| Rotating friction torque | Lower | Higher |
| Heat at speed | Lower | Higher |
| Sensitivity to mounting error | Lower | Higher |
| Cost | Lower | Higher |
Why Rotary Tables Often Need Preload
A rotary table carries a workpiece while a cutting tool pushes against it. Any free play in the bearing lets the table tilt or shift under that force. The result can be:
- Poor surface finish or chatter marks
- Positioning errors after indexing
- Uneven wear on the raceway over time
Removing the play improves stiffness, which is why machine tool builders generally prefer preloaded or zero-clearance designs for precision rotary tables, especially in heavy-cut machines.
The Trade-Offs of Preload
More preload is not always better. Too much can cause:
- High rotating torque, which needs a stronger drive motor
- Heat build-up, especially at higher speeds
- Faster wear if the bearing is over-stressed
- Sensitivity to the mounting surface. A bearing with no play cannot tolerate uneven mounting surfaces the way a loose one can
For low-speed rotation, such as a table that turns only a few revolutions per minute, heat is rarely the issue. Torque and mounting accuracy matter more.
Which Bearing Types Are Used?
Four-point contact ball slewing bearings
They can be built with zero clearance by controlling the ball size relative to the raceway. They suit moderate loads and combined loads in a compact space.
Crossed roller slewing bearings
They have a high stiffness for their size. They are often the first choice when accuracy and rigidity matter most.
For a comparison of the two types, see our guide to rotary table bearings.
How to Decide
| Your situation | Typical choice |
|---|---|
| Heavy intermittent cutting, precision positioning | Preloaded or zero clearance, crossed roller |
| Moderate loads, compact space | Zero clearance four-point contact |
| Low speed, light duty, no precision requirement | Normal clearance |
| Equipment with large temperature swings | Normal clearance, or discuss with the supplier |
If you are unsure, send the supplier your application details: load, speed, accuracy requirement and mounting method. A good supplier will ask for these before recommending a design.
What to Specify When You Request a Quotation
To avoid misunderstandings, state these points in writing:
| 1. Clearance or preload condition | In numbers if possible, and in which direction: axial, radial or both |
| 2. How it will be measured | For example, by dial indicator on a test rig |
| 3. Maximum allowable rotating torque | If your drive is limited |
| 4. Accuracy requirements | Such as runout of the mounting surfaces |
| 5. Mounting details | Including the flatness of your mounting surfaces and the bolt arrangement |
Mounting Matters as Much as the Bearing
A zero-clearance bearing only performs as designed if it sits on flat, rigid surfaces. Uneven surfaces can deform the rings and create tight spots. Follow the supplier's instructions on bolt tightening sequence and torque, and check that the bearing turns smoothly by hand before the machine is put under load.
What We Can Make
Anhui Yuanfeng Slewing Rings Co., Ltd. manufactures slewing rings to customer drawings, including zero-clearance and crossed roller configurations, from 200 mm to 6,000 mm in 42CrMo and 50Mn steel with an induction hardened raceway. Dimensions are verified on our in-house Zeiss coordinate measuring machine, and we hold ISO 9001 and CE certification.
A first or replacement ring can be ordered as a single piece. Production of the first ring establishes an approved drawing that we keep on file. Repeat and batch orders for other machines of the same model run against that same drawing, so no dimensioning work is repeated and every ring after the first is quicker and more economical to produce. Before you order, it helps to know how many machines of that model you run and how many of them will need a bearing, so the first inquiry can be planned as one batch rather than as separate emergency purchases.
Typical production time is around 40 days, with sea freight quoted separately. We need a drawing or detailed parameters to start. We do not offer reverse-engineering or design from scratch, and some configurations are outside our current manufacturing capability. We will tell you at the feasibility stage if yours is one of them.
Frequently Asked Questions
Clearance and preload in slewing bearings for rotary tables
Q: Is zero clearance the same as preload?
Not exactly. Zero clearance means no measurable free play. Preload means the rolling elements are deliberately under light pressure. Agree on the exact condition and how it will be measured before you order.
Q: Does a preloaded bearing wear out faster?
Not if the preload is correct and the bearing is properly mounted. Excessive preload or poor mounting is what shortens life.
Q: Do I need preload for a low-speed table?
Not always. If the table turns slowly, carries a light load and does not need high accuracy, normal clearance can be enough. Preload matters most when cutting forces and positioning accuracy are important.
Q: Can you make a replacement for an existing rotary table bearing?
Yes, if you can give us a drawing or detailed measurements of the old part, including the clearance or preload condition you need. We review feasibility and prepare a drawing for your confirmation before production.
Q: What bearing types suit a machine tool rotary table?
Crossed roller slewing bearings are usually the first choice where rigidity and accuracy matter most, because they are stiff for their size. Four-point contact ball slewing bearings can also be built with zero clearance to suit moderate and combined loads in a compact space.
Q: What information does a supplier need to quote a rotary table bearing?
A drawing or detailed parameters: outer diameter, inner diameter, ring heights, mounting details, gear data if applicable, and your clearance or preload requirement. Stating your load, speed and accuracy target helps the supplier confirm the right configuration.
Choosing a bearing for a rotary table? Leave a message with your drawing or key parameters, including your clearance or preload requirement - our engineers reply within 24 hours.
Editor & Layout: Daphne Hsu
Anhui Yuanfeng Slewing Rings Co., Ltd.
CE & ISO 9001 Certified Manufacturer
Web: www.ahyfeng.com | Email: office@ahyfeng.com / Daphne@ahyfeng.com
Tel: +86-18298009271 | WhatsApp: +86 13965019293
