Linear guide preload is an internal load applied between the rolling elements and raceways before the guide carries an external working load. It reduces internal clearance and increases rigidity, but it also raises movement resistance and makes the guide more sensitive to installation error.
For most profile rail linear guides, preload is established during manufacturing by matching the rail, block, raceways and steel balls. It is not normally adjusted on the machine by tightening mounting bolts, inserting shims between the block and rail or measuring the gap with a feeler gauge.
Before attempting any adjustment, it is therefore necessary to determine whether the problem comes from the internal preload or from rail misalignment, mounting-surface error, contamination, lubrication or an unsuitable preload grade.
What Is Linear Guide Preload?
A recirculating linear ball guide contains steel balls that roll between the block and rail raceways. When balls are selected to produce a controlled interference rather than free internal clearance, the balls remain loaded even before an external force is applied.
This condition is called preload. Its main effects include:
- Reduced internal clearance
- Higher rigidity under changing loads
- Smaller elastic displacement
- More stable response during load reversal
- Greater resistance to vibration and impact in suitable applications
The disadvantage is that higher preload also increases friction, starting resistance, heat generation and sensitivity to mounting errors. More preload is therefore not automatically better.
Can Linear Guide Preload Be Adjusted?
For most standard profile linear guides, the user cannot directly adjust the internal preload after installation.
The preload is normally created by the manufacturer through:
Selection of steel ball diameter
Controlled raceway dimensions
Matching of the block and rail
Assembly and running-resistance inspection
Specified preload class
Changing the internal preload usually requires disassembling the block and replacing or rematching the balls. This should only be performed by the manufacturer or a qualified service facility with the correct ball-size grading, assembly tools and inspection procedures.
What Does Not Adjust Internal Preload?
Tightening the mounting bolts
Mounting bolts secure the rail and blocks to their reference surfaces. Correct bolt torque is necessary for reliable installation, but it does not directly set the internal preload between the balls and raceways.
Excessive or uneven tightening may distort the rail, block or mounting surface and create abnormal resistance that can be mistaken for high preload.
Inserting shims between the rail and block
A shim must never be inserted between a profile rail and its recirculating block. The block must run directly on the matched rail raceways.
Precision shims may sometimes be used under approved machine mounting surfaces to correct height or alignment, but this changes the installation geometry-not the internal preload of the guide.
Using a feeler gauge between the rail and slider
The internal ball-to-raceway contact is enclosed inside the block and cannot be evaluated by inserting a feeler gauge between the exterior surfaces of the rail and block.
Preload condition is evaluated using specified displacement, load or running-resistance methods rather than an external visible gap.
When Preload Can Be Changed
Although most users cannot adjust an installed block internally, the preload level can be changed in several controlled ways.
Order a different preload grade: Select a light, medium or heavier preload class according to the manufacturer's available specifications.
Replace the rail-and-block assembly: Use a guide supplied with the required preload when the current grade does not suit the machine.
Return the assembly for rematching: The manufacturer may replace or rematch the balls when the guide design and condition permit.
Use a designed external adjustment mechanism: Some special guide arrangements have adjustable gibs, opposing bearings or manufacturer-specified adjustment screws. These mechanisms should be adjusted only according to the equipment drawing.
An external guideway adjustment mechanism should not be assumed to change the internal preload of an individual profile rail block.
Original Linear Guide Images
Micro linear guides require suitable preload without introducing excessive movement resistance.
The preload of a profile linear ball guide is normally established during block assembly.
How to Check Whether Preload Is Too High
A guide with excessive internal load or installation-induced stress may show:
High movement resistance over the full stroke
High starting force
Abnormal temperature rise
Motor current higher than expected
Reduced maximum speed
Accelerated raceway or ball wear
These symptoms do not prove that the factory preload is incorrect. Similar resistance can be caused by poor rail parallelism, uneven mounting surfaces, damaged seals, excessive lubrication, contamination or block distortion.
How to Check Whether Preload Is Too Low
Insufficient preload or excessive internal clearance may appear as:
Movement or vibration under reversing load
Reduced carriage rigidity
Unstable positioning under changing moment load
Unexpected roll, pitch or yaw displacement
Noise caused by looseness or impact under changing load
Before replacing the guide with a higher preload class, check the block quantity, rail spacing, load direction and moment-load arrangement. Insufficient system rigidity is not always caused by the preload grade.
Diagnostic Procedure Before Replacing the Preload Grade
Step 1: Confirm the original specification
Check the rail and block model, accuracy class and preload designation. Do not determine the preload grade only from how the block feels when moved by hand.
Step 2: Clean the rail
Remove dust, metal particles, dried grease and other contaminants from the exposed rail surfaces. Do not flush unknown solvent directly into the block unless the manufacturer permits it.
Step 3: Check lubrication
Confirm that the lubricant is suitable and that the amount is appropriate. Insufficient lubrication increases friction and wear, while excessive or unsuitable grease can temporarily increase running resistance.
Step 4: Check mounting surfaces
Inspect for burrs, paint, chips and dents beneath the rail and block mounting surfaces. Even a small contaminant can distort the assembly after the bolts are tightened.
Step 5: Check rail parallelism
In a two-rail system, excessive parallelism error creates internal side load. A higher-preload block has less freedom to accommodate this error and may bind more noticeably.
Step 6: Check tightening sequence and torque
Tighten the rail and block bolts progressively according to the specified sequence and torque. If movement resistance changes substantially after final tightening, inspect the reference surfaces and alignment.
Step 7: Measure running resistance
Use a suitable force gauge or controlled drive measurement to compare movement resistance along the complete stroke. A uniformly high resistance may point toward preload, seals or lubrication. Resistance concentrated at one position is more consistent with contamination, rail damage or installation error.
Do Not Remove the Block Without a Transfer Rail
Some recirculating blocks include ball retainers, but this does not mean that every block can be safely removed from its rail without preparation.
If removal is necessary:
Use the correct transfer or temporary rail.
Keep the original block orientation and rail position identified.
Do not allow the balls or seals to contact a dirty surface.
Do not interchange matched blocks and rails without confirmation.
Do not replace balls individually in an attempt to adjust preload.
Choosing an Appropriate Preload Level
The correct preload depends on the required rigidity, load, speed, accuracy and installation conditions.
| Preload direction | Typical priority | Important limitation |
|---|---|---|
| Clearance or very light preload | Low resistance and economical general motion | Lower rigidity under reversing or moment loads |
| Light preload | Balanced smoothness, rigidity and service life | Still requires accurate rail installation |
| Medium or higher preload | Higher rigidity and resistance to vibration | Higher friction, heat and sensitivity to misalignment |
Preload codes and percentages are not universal. Always use the definitions given for the selected manufacturer and series.
Conclusion
The internal preload of most linear ball guides is set during manufacturing and should not be adjusted on the machine by inserting shims, changing mounting-bolt torque or measuring an external gap with a feeler gauge.
If a guide feels too tight or too loose, first inspect lubrication, contamination, mounting-surface accuracy, rail parallelism and bolt tightening. When a different preload is genuinely required, the safest solution is normally to order the appropriate preload grade or return the assembly for professional inspection and rematching.
View DLY's linear guideway range or read the related guide to linear guide preload levels and selection.
