WORK ORDER — Maintenance and Improvement Record
Work order no: WO-A-2026-07 Date issued: 2026-09-11 Category: Material handling
Machine / cell: Two-station scissor lifting traverse ST-2 between the machining line and the wash cell
Status: Closed Engineer: P. Novak (automation) Total labour: 7.0 hours Parts cost: EUR 92.00
Reason for work: the lifting traverse drifts sideways while raising a loaded pallet and the payload rocks
1. Fault description as logged by the shift team
Wednesday 08:20, operator record: the scissor lifting traverse raises the 250 kg loaded pallet from the line height of 850 mm to the wash entry at 1,150 mm, traverses it sideways and lowers it into the wash position, and in the last two weeks the pallet rocks sideways through the last 60 mm of lift, the far corner lags about 18 mm behind the near corner, and on one occasion the pallet edge clipped the wash entry sill and shaved a chunk out of the rubber bumper. The team had re-seated the traverse rails and re-greased both scissor pivots, and the rock came back on the third shift, which pointed away from the rails and toward the linkage geometry doing the lifting.
The traverse is a single scissor frame on a cross-slide, the scissor is driven by a lead screw through a nut bracket, and the whole frame slides sideways on linear rails via a second belt drive. The two halves of the scissor must stay mirror-symmetric as they fold, because a scissor that folds unevenly lifts one side higher and tips the platform, and the platform itself carries the two stations side by side, so the operator sees the rock as a pallet fault when it is really a linkage fault. I stripped the scissor covers and laid the dial indicators on both platform corners before I moved a single screw, because a rocking scissor leak is best caught at the geometry before it becomes a collision.
Evening note to the file: a scissor lift that rocks is a machine telling you its two legs have stopped trusting each other, and the legs have not learned a new trick on their own, something in the linkage loosed its grip and let one leg walk.
2. On-site inspection record (Day 1)
I lowered the empty platform to the bottom stop and clamped the two dial indicators vertically on the near and far platform corners, then raised the traverse slowly and watched both needles. The near corner rose smoothly for the whole stroke, and the far corner lagged from mid-stroke onward, showing 18 mm of lag at the top, so the platform was not tipping as a rigid body but being lifted unevenly by a linkage that had gone asymmetric. I traced the asymmetry to the drive side: the lead screw nut bracket had worn its thread engagement on one flank, the nut was riding loose on the screw by about 0.4 mm of axial play, and that play loaded up only when the screw reversed direction at mid-stroke to flip the scissor past its centre geometry, after which the loose leg pushed the far corner slower and the near corner carried the lot.
I also found the two scissor pivot pins worn unevenly, the near pin 0.15 mm undersize and the far pin 0.30 mm undersize, which means the far leg had been carrying the slop of the whole linkage and wearing its own pin to match. The cross-slide rails themselves were within specification, 0.02 mm over a metre straightness, which cleared the rails as the team suspected and put the whole fault on the linkage drive, exactly where the earlier re-grease had not touched. I measured the platform rock with the loaded pallet in place to confirm the empty-platform numbers, and the loaded case showed the same 18 mm far-corner lag at the top stroke plus a distinct knock when the scissor passed its centre, the knock being the loose nut bracket reversing its play under load.
Evening note: the rock was not the platform and not the rails, it was one worn nut flank and two asymmetric pins, three small clearance debts that the linkage was paying back as an 18 mm tip at the top of every lift.
3. Corrective action taken
I replaced the lead screw nut with the maker’s anti-backlash nut, which preloads the flank engagement and removes the 0.4 mm axial play at direction reversal, so the two scissor legs now move as one driven pair past centre instead of one leg walking. I replaced both scissor pivot pins with the hardened replacement pair and re-bushed the far leg to take the wear case out of the linkage, fitting the bush so the pin ran true and the leg folded on the drawing pivot rather than on the worn hole. I re-torqued all four scissor pivot bolts to the drawing 45 N·m, because a pivot that runs loose at the bolt can read on the platform as a leg lag, and I set the platform corner-to-corner level to within 0.5 mm at both the 850 mm line height and the 1,150 mm wash height with the frame shimmed at the base.
After the linkage rebuild I ran the traverse empty, then with a 250 kg loaded pallet, for thirty-five full cycles, and the platform rose dead level on every cycle, the far-corner dial indicator holding within 0.4 mm of the near corner for the whole stroke, against the 18 mm lag before, with zero knock at direction reversal because the anti-backlash nut took the play out of the drive. I re-checked the traverse position against the wash entry sill and the clearance is now a uniform 6 mm all along the sill, so the bumper has a real gap to work with instead of a contact that waited for the right moment.
| Item | Before | After | Spec |
|---|---|---|---|
| Far-corner lag at top stroke | 18 mm | 0.4 mm | ≤0.8 |
| Nut axial play at reversal | 0.4 mm | preloaded, 0.02 mm | ≤0.05 |
| Pivot pin wear | 0.15 / 0.30 mm | new pair + bush | ≤0.05 |
| Sill clearance | contact, bumper shaved | 6 mm uniform | ≥4 |
The station duty sheet now lists the anti-backlash nut and the pivot pins on a half-yearly inspection, with the two dial-indicator check logged as the acceptance test, because a linkage that turns an 18 mm tip into a collision deserves a number in the sheet, not a memory on some floor.
4. Acceptance result after the repair
I left the traverse running the production programme after the rebuild, twenty-four pallets a day across both heights, and over four shifts the platform stayed level on every lift, the rock gone entirely, the knock gone from mid-stroke and the wash entry sill untouched, with the rubber bumper back to its original profile and no new witness marks anywhere. The wash cell stopped seeing the tilted pallet that once made the wash nozzles spray into a skewed part, quality noted the improved wash coverage on the far side of the pallet that the rock had been robbing, and the line returned to the normal takt with no collision stops on the transfer. I closed the order with the traverse released to production and the dial-indicator check written into the monthly duty sheet, so the next level reading is a scheduled number rather than a crash report.
5. Hours and parts list
- Labour — 7.0 h total: platform dial-indicator survey 2.0 h, nut and pin replacement 2.0 h, pivot re-torque and platform level 1.5 h, loaded cycle verification 1.5 h
- Parts — anti-backlash nut and collar EUR 34.00, hardened pivot pin pair EUR 29.00, re-bush and pivot bolts EUR 24.00, frame shims and grease EUR 5.00; total EUR 92.00
- Dysfunction avoided — rock and contact on every lift before repair, zero after; no pallet or cell damage in the acceptance window
6. Retrospective notes for the next engineer
When a scissor traverse rocks, measure the two platform corners independently before you touch anything, because a rocking platform with a level rail is a linkage telling you its legs disagree, and in a scissor the legs can only disagree through the drive or the pins. The 0.4 mm of nut play at direction reversal and the 0.30 mm far-pin wear were both under a millimetre, yet they paid out as an 18 mm tip at the top stroke, which is the whole lesson: a scissor is a mechanical amplifier, small clearance debt in the drive comes back huge at the platform edge, so the anti-backlash nut is not a luxury on a loaded traverse, it is the device that keeps the two legs moving as one through centre. Put the two-corner dial check on the duty sheet, run it before the bumper finds out, and treat any knock at reversal as the sound of a nut about to walk, because a traverse that lifts level, every time, is carrying the line’s pallets and its patience in equal measure.
Attachments: two-corner dial indicator log, nut and pin measurement sheets, loaded-cycle verification record, wash coverage note (filed with WO-A-2026-07).
Glossary — scissor lift: a folding linkage that raises a platform; traverse: the sideways slide of the platform between stations; anti-backlash nut: a nut preloaded to remove thread play at reversal; pivot pin: the pin at each scissor hinge; platform rock: the uneven rise of the platform corners.
Field note — why a scissor leg gets to disagree and how the arithmetic works
The geometry behind this fault deserves a permanent note because it explains the 18 mm number. A single scissor frame is two equal legs pinned at the centre and hinged at the base and the platform, so the platform stays parallel to the base only as long as both legs fold by the same angle from the same starting symmetry. The drive enters through one nut on one leg, and that is the vulnerable point: with 0.4 mm of axial play in the nut, the driven leg reverses direction at mid-stroke a fraction later than the free leg, so for a short arc the free leg folds ahead, the platform tilts, and the far corner lags. Amplify that angular lag by the platform half-span, which here is 900 mm, and a leg angular error of just over a degree at the top stroke works out to the measured 18 mm of far-corner lift shortfall, a small thread play turned into a visible rock by the lever arm of the whole platform. The anti-backlash nut removes the play at its source, and the hardened pins remove the wear asymmetry that let the far leg carry extra slop, which is why the pair fixes a fault neither could fix alone. Design takeaway for anyone quoting a new traverse: give the scissor a preloaded drive nut as standard, specify the pivot pin hardness against the loaded leg, and draw the platform parallel check as an acceptance line, because a 250 kg pallet is entitled to a level lift, and the linkage, not the rail, is what owes it the level.
One field habit worth copying: after any linkage repair, cycle the traverse once to bottom stop and once to top stop with the two dial indicators still clamped, and write the two numbers side by side in the sheet. The reason is that a loose nut and a worn pin can cancel each other at one height and show nothing, but they always separate at the extremes, so a two-corner reading at both stops is the cheapest way to prove the legs agree again. The shift I handed over ran with this check on the first cycle of every morning, a thirty-second habit that catches a half-millimetre of play years before it becomes a bumper shaving, and that is the kind of habit a station earns after one good scare.
