Technical Guide
Slideway Liner Creep and Permanent Set Under Sustained Load
A PTFE slideway liner that measures correctly on the day of a rebuild can look different after the machine has carried load for a month. This guide explains the creep and permanent-set behaviour that belongs to any PTFE-based liner and its bonded joint, how to separate recoverable compression from real set, and how to record both so a later change is measured instead of guessed.
Direct answer: a liner settles, and the rate falls away
A PTFE-based slideway liner is a viscoelastic material, and a viscoelastic material continues to deform over time under a standing load even when nothing moves. For a rebuilder the practical answer is that most of the movement happens early, the rate then falls away, and the question worth answering is not whether the liner settles but how much it has settled against the baseline that was recorded. That answer needs no quoted constant: measure the finished liner and the centre height at defined stages, keep every reading, and compare later readings at the same stations and the same temperature.
Creep is not the same as compression
Compression describes the deformation produced while a load is applied. Creep describes the additional deformation that accumulates while the same load stays applied. An axis that traverses continuously may never sit long enough for the second effect to become visible, while an axis that is clamped, parked, counterweighted or loaded by a stationary workpiece can hold the same pressure on the same strip positions for hours. That difference in duty is why two machines fitted with the same liner at the same measured pressure can show different finished geometry a month after the rebuild.
| Load case | What keeps the load applied | Where it shows | What to record |
|---|---|---|---|
| Locked or parked axis | Clamp, brake or servo hold with no movement | The same strip positions loaded for hours | Centre height at the park station with a timestamp |
| Clamped workpiece on the slide | Fixture pressure passed through table and saddle | Table ways under the fixture footprint | Fixture mass, clamping time and contact pattern |
| Counterweighted or vertical member | Counterweight holds a standing load on one face | The loaded face of the column or vertical slide | Loaded face, oil retention and drain-back at that face |
| Tailstock, quill or stop held on the work | Continuous thrust with no traverse | The way section carrying that thrust | Thrust direction, contact area and position along travel |
| Grinding or milling cycle with a long dwell | Dwell time concentrated at one end of a short stroke | The dwell end of a reciprocating way | Dwell duration and the oil state at both ends |
Recoverable compression or permanent set?
Not every change after a rebuild is permanent. Part of the deformation in a PTFE-based liner recovers when the load is released and part of it may not. The way to separate the two is to take more than one reading under the same conditions, including at least one reading taken after the slide has stood unloaded long enough for the recovered part to return.
| Observation | More likely recoverable | More likely permanent | First action |
|---|---|---|---|
| Reading returns towards the as-machined value after the load is released | Yes | Not this case | Repeat at the same station and temperature |
| Reading steps down once, then holds steady on later checks | No | Yes | Record the new baseline, then re-check contact and clearance |
| Reading keeps moving at every check with no stabilising trend | Inconclusive | Inconclusive | Verify temperature, oil film, datum and instrument first |
| Change is local to one station with a visible edge or step | No | Mechanical, not creep | Inspect the bond edge, contamination and the prepared way |
| Change tracks the thermal map of the machine | Not a liner set | Not a liner set | Compare with the cold and warm record before changing thickness |
The bonded joint and the oil film are part of the measurement
A relined way is a stack: metal way, adhesive layer, liner, oil film and mating member. A thick or uneven adhesive layer settles differently from a thin uniform one, and under a standing load the oil film can be squeezed out of the contact at rest, which changes the drag the operator feels without changing the liner thickness at all. Measure the assembly rather than the strip alone, and keep bond-line quality in the record. TEBAK supplies the two-part epoxy adhesive as the bonding component for its liner range, which covers Blue TEBAK slideway liner, Brown TEBAK slideway liner and the other colours in the same compound.
A set measurement card for the workshop
Use the same stations, the same datum and the same instrument for every line of the card, and write down the temperature at the time of each reading. A dial indicator on a magnetic base, a micrometer, and a surface plate with a known reference block are enough for most rebuilds. The value of the card comes from repeating the method, not from the resolution of the instrument.
| Stage | Condition | Measure | Record |
|---|---|---|---|
| As supplied | Strip before any bonding | Thickness at marked stations | Station, temperature and instrument |
| After bonding and cure | Adhesive cured, running face unmachined | Thickness at the same stations | Cure time and workshop conditions |
| After finish machining | Running face at finished geometry | Thickness and centre height at the datum | Finished geometry against the drawing or repair record |
| First loaded check | After a defined period under the real duty | Centre height, contact and drag at the same stations | Load case, time under load, temperature and oil state |
| Released check | After the slide has stood unloaded | Centre height at the same stations | Time unloaded and whether the reading recovered |
Engineering judgement: what to do with the result
- Keep contact pressure inside the published design range for the liner and apply the load to the real bearing lands, not to the nominal tape width. The slideway liner load and pressure guide sets out the static and dynamic ranges the TEBAK liner is specified for.
- Do not answer a set reading by tightening the gib or by ordering a thicker liner. Thickness sets finished geometry, not pressure, and tightening clearance simply moves the problem to another point on the travel.
- Reduce the pressure instead: increase the effective bearing area, remove the cause of a concentrated load, or correct the geometry that produced a local high spot.
- If the change is smooth, repeatable and still inside the machine acceptance band, record it as an operating condition rather than treating it as a defect.
- Agree the acceptance target before the rebuild, so the loaded reading has something to be compared with. The checks described in the accuracy after relining guide are the same measurements applied once the machine is back in service.
Where creep fits with the other checks
Set behaviour, thermal movement and load pressure interact, and a reading taken soon after the machine warms up can contain both creep and thermal movement. That is why the guideway thermal expansion check uses the same station-based method. Work the three checks from one record so that a single reading is never used to justify a permanent correction.
Frequently asked questions
Does a PTFE-based liner keep settling indefinitely?
The creep rate falls with time once the load is steady, so the useful measurement is the change between two defined checks rather than an assumed end point. Record the trend instead of predicting a final figure.
Can permanent set be solved by ordering a thicker liner?
No. Thickness sets the finished geometry after machining. If pressure is the cause, correct the load path or increase the effective bearing area, then measure again.
Should the gib be tightened if the slide feels loose after a period under load?
Re-measure centre height, contact and clearance first. Adjusting to one transient reading can leave the axis tight at one end of its travel.
Is creep worse on a vertical face than on a horizontal way?
The material behaviour is the same, but the duty differs. A vertical or clamped face can hold a standing load with poorer oil retention, so it needs its own recorded check rather than an assumption taken from the horizontal axis.
Send the measurement record with the specification
For a specification review, send the machine model and axis, the way type and bearing width, the load case and how long the load stands, the liner colour and dimensions, the curing record, and the centre-height readings from every stage of the measurement card. The TEBAK team reviews that record against the product data and confirms the liner and adhesive for the job. Begin with the Blue TEBAK slideway liner and the matching two-part epoxy adhesive, or ask which colour and thickness suits the measured allowance on your way.