Technical Guide
Where a PTFE Slideway Liner Is the Wrong Choice: Application Limits
A bonded PTFE slideway liner solves a friction problem, and a rebuild that is really a stiffness, speed or contamination problem will not be solved by one. This guide sets out the four conditions that decide whether a liner belongs on a given axis, the pressure and speed figures that mark the edge of the range, and the alternative repair to use when the liner is the wrong answer.
Direct answer: a liner changes friction, and only friction
A soft liner works by putting a low-friction, self-lubricating running face between two metal surfaces. That single change removes stick-slip, protects the way underneath and restores a worn centre height. It does not raise the stiffness of the slide, it does not carry load that the bearing land cannot support, and it does not survive an abrasive that is allowed to pack onto the running face. When the fault on the machine is one of those three things, a relined axis will reproduce the original complaint within a few weeks of service.
The practical screen is therefore short. Check the calculated bearing pressure, the speed and duty cycle, the orientation of the face and what reaches the running surface. Four readings either clear the job for a liner or send it to a different repair, and it is cheaper to take them before the strip is ordered than after the machine is back together.
The four conditions that decide the application
| Condition | Where a bonded liner fits | Where it stops fitting | Alternative repair |
|---|---|---|---|
| Bearing pressure | Calculated at or below the general machinery figure of about 5 MPa, against published liner limits of 10 to 12 MPa static and 6 to 8 MPa dynamic | Calculated pressure lands in or above the 8 to 10 MPa band, or the real contact area is a fraction of the nominal land | Widen the bearing land, fit a replaceable wear plate, or move the axis to a rolling element |
| Sliding speed and duty | Intermittent traverses and feeds of the kind a knee mill, lathe saddle or surface grinder table runs | Continuous travel at rapid rates for hours, where the same strip positions are worked without pause | Recirculating linear guide, roller guide or hydrostatic support |
| Orientation of the face | Horizontal or moderately inclined faces where the strip sits in compression and is positively located | Vertical, overhung or inverted faces where the strip hangs in shear and edge load cannot be avoided | Mechanical retention such as dovetail strip, screwed wear plate, or a different bearing arrangement |
| What reaches the face | Flood coolant, way oil and swarf that is wiped or washed clear | Dry cutting of cast iron, or abrasive dust that packs the way and is then dragged | Way covers, wipers, air purge, or a hardened and ground way instead of a soft liner |
Pressure: the limit most rebuilds actually cross
The load on a slideway is carried by the area that is really in contact, and that area is almost never the full nominal strip. A 300 mm wide strip bonded to a way that bears on 180 mm of land is a 180 mm bearing, and a strip whose contact pattern shows bearing on two edges is narrower again. Divide the load by the measured area rather than the ordered width. The slideway liner load and pressure guide sets out the static, dynamic and interrupted figures and the reasoning behind them, and the same guide gives the worked check of load divided by area against the 8 MPa general machinery band.
Load is also not constant along the travel. A table with the workpiece at one end, a ram extended, or a counterweight hanging on a vertical member all move the load onto a different part of the strip. If the pressure check passes only when the load is centred, the axis is outside the comfortable range for a soft liner even though the average figure looks acceptable.
Speed, duty and the film that has to survive
A soft liner runs on a boundary or mixed film, and the lubrication between the strip and the mating way is what keeps wear and friction stable. An axis that traverses occasionally lets the oil re-form between passes. An axis that runs continuously at high speed works the same strip positions without recovery time, and the film becomes the limiting component rather than the liner. The same reasoning is behind slideway lubrication and oil groove design: more oil grooves do not rescue a duty cycle the material was not chosen for.
Orientation and retention
- On a horizontal face the strip sits in compression and the adhesive carries shear only at the edges, which is the condition the bonded joint is designed for.
- On a vertical face the strip is loaded across the bond line and any edge that lifts becomes a starting point for peeling, so a vertical axis needs both a proven bond and a mechanical stop.
- On an inverted or overhead face gravity works against the bond and swarf falls onto the running surface, so retention and protection matter more than the friction figure.
- On a dovetail the angled face carries load in a direction that pushes the strip towards its free edge, which is why the gib clearance calculation has to be settled before any strip is bonded to a dovetail gib.
Stiffness and precision: the trade a liner makes
A polymer running face is more compliant than cast iron on cast iron. That compliance is what damps chatter and absorbs the friction change at low speed, and it is also why a liner is not the route to sub-micron positioning on a machine that needs high dynamic stiffness. Where the specification demands that order of precision, the answer is usually a different bearing type rather than a thinner strip. Where the specification is a normal machine tool tolerance and the complaint is hesitation, waviness or a worn centre height, the liner is the cheaper and more predictable repair.
Screening three common jobs
| Job | Reported symptom | Verdict on a liner | Reason |
|---|---|---|---|
| Vertical milling machine, knee and saddle ways | Table hesitates at slow feed, finish shows waviness, centre height has dropped after years of wear | Fits | Horizontal faces, intermittent duty, pressure inside the general machinery band once the bearing land is measured |
| Surface grinder, table ways | Table jumps at very slow traverse, wheel marks appear at reversal | Fits, with a duty check | Very low speed suits the material, but the pressure and the oil film have to be confirmed because the table is heavy and the bearing land is narrow; see the grinder guideway rebuild guide |
| Vertical ram or quill slide with an overhung load | Strip lifts at the lower edge, bond fails at the ends, axis loses position under cut | Does not fit as a straight substitution | The face is vertical and overhung, so the load pulls across the bond line; retention and geometry have to change first |
Frequently asked questions
Can a slideway liner be used on a vertical axis?
It can be bonded to a vertical face, but the load then acts across the bond line rather than compressing the strip, so edge lifting and peel become the failure mode. A vertical axis needs mechanical retention and a bond that has been proven on that geometry, not just a strip selected by thickness.
Is a thicker liner better for a heavily loaded slide?
No. Thickness changes the geometry and the compliance, not the allowable pressure, which depends on the contact area. A thicker strip on an overloaded land increases the movement under load and makes the pressure problem worse rather than better.
Can a liner be fitted to a machine that cuts cast iron dry?
The material survives cast iron dust, but the running face does not survive dust that is allowed to pack and then be dragged. Without way covers, wipers or an air purge the abrasive reaches the film and the wear rate becomes unpredictable, so the protection is part of the decision.
When should the axis move to a linear guide instead?
When the duty is continuous high-speed travel, when the required stiffness is beyond what a polymer face can hold, or when the pressure cannot be brought into range by widening the land. Those are bearing-type decisions, and no thickness of soft tape answers them.
Screen the axis before choosing the strip
Send the machine make, model and axis, the measured bearing land rather than the ordered strip width, the load the face carries with the workpiece in its worst position, the fastest and slowest feed the axis runs, the orientation of the face, and what reaches the running surface. With those five items the pressure check and the duty check can be settled before anything is ordered. The TEBAK range covers the Blue PTFE slideway liner and the other colours in the same compound, the matching two-part epoxy adhesive for the bond, and the Tuschierpaste used to read the contact pattern that decides the real bearing area, so the screen, the bond and the contact check can be specified as one job.