WORK ORDER — Maintenance and Improvement Record
Work order no: WO-A-2026-06 Date issued: 2026-09-11 Category: Component feeding
Machine / cell: Feed and Sorting Station FS-3 upstream of the assembly line
Status: Closed Engineer: P. Novak (automation) Total labour: 6.0 hours Parts cost: EUR 74.00
Reason for work: magazine ejector stalls and jams on every third or fourth batch of bushings
1. Fault description as logged by the shift team
Tuesday 10:10, operator record: the feed station loads 30 mm cylindrical bushings from a vertical stack magazine onto the sorting conveyor, and every third or fourth batch the ejector stroke stalls part way, the downstream gripper arrives at an empty pick pocket and the line timer trips a cycle fault, costing about four minutes of downtime each jam and a box of parts momentarily out of position. The team had changed the ejector cylinder, then the solenoid valve, on the theory that the air actuation was the weak link, and each change helped for a day and then the jam came back, which is the fingerprint of a mechanical binding that only shows when the stack reaches a certain height or a certain part mix.
The magazine is a simple gravity stack, bushings dropped from the top chute onto a pair of rails, and a pneumatically driven ejector pusher slides the bottom bushing out through a throat onto the transfer pocket. The parts are the same nominal bushing but their chamfer, length and surface finish vary slightly between batches, because they come from two different suppliers, and a stack magazine is extremely sensitive to that mix, a part that is 0.1 mm longer or without a chamfer changes how the stack beds at the throat. I brought callipers, a stack-height gauge and a video camera rather than the parts bin, because a jam that follows batch changes is usually a geometry or feed-control problem, not an air problem.
Evening note to the file: the second time a station owner changes the cylinder for the same jam, the fault has stopped being an actuator and started being a feeder, and the feeder geometry does not get better by swapping the thing that pushes.
2. On-site inspection record (Day 1)
I ran the station with a fresh mixed batch and filmed the jam in slow motion, and the recording showed the sequence nobody saw live: as the bottom bushing started to slide out, the second bushing in the stack tilted forward, caught its lower corner on the throat edge, and rode down on top of the ejector head, jamming the pusher before it reached the transfer pocket. The ejector was pushing two bushings, not one, because the throat gap had grown enough to admit the second part’s corner while the stack weight and the part mix decided how far the second bushing could lean. I measured the throat opening at 31.2 mm against the drawing 30.4 mm, a 0.8 mm wear growth over four years of sliding steel bushings, and the ejector head itself had worn 0.3 mm on its top leading corner, so the head no longer shaved the stack cleanly and instead lifted the second bushing onto its back.
I measured a sample of the two suppliers’ bushings while the throat was the suspect: supplier B parts ran 0.3 mm longer on average than supplier A, and half of supplier B had a machined chamfer while the rest were edge-broken, and in a stack the chamfer lets the part settle into the throat gap, while the sharp-edged parts catch and tilt. The combination arithmetic was ugly: a 0.8 mm throat wear plus a 0.3 mm longer part plus a sharp edge on the second bushing left almost no room for the ejector to pass cleanly, and any one of the three on its own would have run for weeks, but the three together jammed every third batch and blamed the cylinder.
Evening note: the station was not failing on bad air, it was failing on bad geometry budget, the throat had grown, the parts had grown, and the ejector head had worn, so the feed clearance that once passed one bushing cleanly now passed one bushing and one corner, and the fix had to spend all three debts, not just re-stroke a cylinder.
3. Corrective action taken
I rebuilt the throat as a wear part: I stripped the magazine lower block, machined the wear growth out of the throat and fitted a hardened steel throat insert with the drawing opening of 30.4 mm, so the passage is back to drawing and now carries a replaceable wear surface rather than wearing the block itself. I replaced the ejector head with the hardened version from the maker’s upgrade kit, its leading top corner shaped with a small lifting chamfer that shaves the stack clean and guides the second bushing over the top of the head instead of trapping it, which turns the worst geometric case into a part that simply rides over. I re-greased the ejector guide and checked its squareness to the throat with the square and a feeler, so the head slides true and does not heel against one side of the throat.
On the part side I set a feeder rule instead of an endless battle: the station now sorts incoming bins by supplier into two magazine groups, so the stack is not mixing a 30.2 mm part with a 30.5 mm part mid-stack, and I flagged the sharp-edge supplier B parts to quality, because a stack magazine feeds cleanly only within a length and chamfer band, and a mixed stack spends that band and invites the corner to catch. I tested the rebuilt feeder across three mixed and three segregated batches, forty bushings each, and every part fed singly, with the ejector stroke landing in the pocket on all 240 parts and zero jams, against the previous rate of one jam per thirty to forty parts on a bad batch.
| Item | Before | After | Spec |
|---|---|---|---|
| Throat opening | 31.2 mm | 30.4 mm insert | 30.4 |
| Ejector head top corner | 0.3 mm worn | hardened, lifting chamfer | as new |
| Feed failure rate | 1 per 30-40 parts | 0 per 240 parts | <1:200 |
| Part stack mix | two suppliers mixed | segregated by supplier | uniform |
The station duty sheet now lists the throat insert wear at the buffer-change interval, because a feeding station that wore its throat 0.8 mm in four years will wear the insert faster and the insert is the check-point that catches the growth before the corner catches the part.
4. Acceptance result after the repair
I ran the rebuilt feeder across five full production shifts, roughly 9,000 bushings through the magazine, and the ejector did not jam once, against the prior three to four stalls per shift on the worst mix. The downstream gripper found a part in the pick pocket on every cycle, the line timer stopped tripping cycle faults, and the four-minute-average downtime per jam disappeared from the station log entirely. Quality signed off the segregated feeding rule, and the supplier B sharp-edge parts are being reworked with a chamfer at their source, so the band the magazine needs is fixed at the part rather than fought at the station. I closed the order with the feeder released, the throat insert logged for wear, and a one-page note pinned to the station explaining that a stack feeder jams on geometry budget, not on air.
5. Hours and parts list
- Labour — 6.0 h total: slow-motion filming and throat inspection 2.0 h, throat insert machining and fit 1.5 h, ejector head and guide service 1.0 h, feed tests and shift verification 1.5 h
- Parts — hardened throat insert EUR 28.00, hardened ejector head EUR 31.00, guide grease and fasteners EUR 15.00; total EUR 74.00
- Downtime avoided after fix — roughly 30 hours of jam stop per month at the old rate, measured from the shift log
6. Retrospective notes for the next engineer
When a feeder jam comes back after a new cylinder and a new valve, stop changing actuators and start measuring the geometry, because the slow-motion film told the whole story in five seconds, the second bushing tilted forward and rode on the ejector head because the throat had grown 0.8 mm and the head had worn 0.3 mm and the parts had grown 0.3 mm, three small numbers that together ate the feed clearance. A vertical stack magazine is a precision device dressed as a box of parts, its chamfers, lengths and edge conditions do the separating, and mixing suppliers in the stack hands the corner a lever to catch. Buy the hardened throat insert and the lifting-chamfer ejector head, run the stack in uniform bands, and put the insert on the wear check, because a feeder that feeds one bushing at a time, quietly, all day, is the cheapest reliability a line can own.
Attachments: slow-motion jam recording stills, throat and ejector measurement sheet, supplier part dimension survey, five-shift feed log (filed with WO-A-2026-06).
Glossary — magazine: a stack holder feeding parts one at a time; ejector: the pusher sliding the bottom part out; throat: the opening the part passes through; stack chamfer: the edge relief that lets a part seat cleanly in the stack; pick pocket: the transfer station where the gripper collects the part.
Field note — the feed-clearance budget a stack magazine actually needs
Worth a paragraph on how the numbers in this job fit together, because the same arithmetic applies to any stack feeder. The ejector must pass the bottom bushing while the parts above it stay put, so the design consumes three clearances at once: the throat opening must clear the largest part diameter, the second-bushing tilt must stay below the point where its corner enters the throat, and the ejector head must shave the stack with its own controlled chamfer rather than lift the part above. Each of those clearances started on the drawing at a few tenths of a millimetre, which is the whole tolerance budget of the feeder, and four years of wear spent the budget: the throat grew 0.8 mm, the parts gained 0.3 mm between suppliers, and the head lost 0.3 mm, so the remaining budget went negative exactly where the second bushing tilts, and the corner caught. The durable answer is the one used here, make the throat a replaceable hardened insert at the drawing opening, give the ejector head a lifting chamfer so a tall part rides over instead of catching, and run the stack in a length and chamfer band the feeder can actually separate. Designers note this too: a stack feeder drawing should quote the maximum part dimensional band the feeder will accept, not just the nominal part, because a station built for a nominal 30.0 mm bushing that quietly tolerates supplier variation up to 30.5 mm is the station that jams exactly on the batches that strain the band. And maintenance, film the jam, because five seconds of slow motion beats three days of swapping cylinders, and the film shows the geometry the parts never complain about in words.
One more note that surfaces in the design review every time: the ejector stroke length and the throat-to-pocket offset are a matched pair. I checked the transfer pocket position against the ejector home stroke after the rebuild, and the pocket was fine at the drawing 34 mm centre distance, but the sheet shows how easily a bump to the pick height, of even a millimetre, makes the pusher deliver the bushing short, and the gripper then misses a part that visually arrived. Keep the pocket height and the ejector home on the same datum, measure both when the feeder is stripped, and log them with the throat insert, so the next repair starts from known geometry instead of a whole new sensor hunt.
