The Pallet That Wouldn’t Locate on the Return Conveyor
We built a pallet circulation system: pallets carry parts through assembly stations. The pallets locate on each station with two pins. On the outgoing conveyor, the pallet went through fine. On the return conveyor, it came back rotated 180°. The pins didn’t line up with the station’s bushings. The problem: the pallet had no orientation feature (it was symmetric top-bottom). When it flipped on the return, the pins were 180° off. We added a key (a slot on one side) that only mates one way. The pallet always came back oriented correctly. The mistake was making the pallet symmetric — no orientation feature.
Pallet and fixture design for circulation systems is about repeatable location and orientation. The pallet must locate the same way every time it reaches a station. This article covers the design.
What a Circulation Pallet Does
A circulation pallet carries a part through multiple stations (assembly, test, packaging). The pallet rides on a conveyor. At each station, it stops and locates (pins drop into bushings). The part is fixed. Work happens. Then the pallet moves on.
The pallet’s job: hold the part consistently. Every time it reaches a station, the part is in the same position.
Step 1: Pallet Locating (Datum)
The pallet locates on station stops. Standard method: two pins (article 69) — one round, one diamond. The pins drop from below (or the pallet lowers onto the pins).
- Round pin: Locates X and Y (two directions).
- Diamond pin: Locates rotation (but allows hole-to-hole distance variation).
- Bushings in the pallet: The pallet has hardened bushings that the pins enter. The bushings are wear surfaces (replaceable).
The pins are on the station (fixed). The pallet has the bushings (replace when worn). Don’t harden the pallet’s locating holes directly — use replaceable bushings.
Step 2: Pallet Orientation
The pallet must always arrive at the station in the same orientation. If it flips (180°), the pins miss the bushings.
- Non-symmetric pallet: The pallet has a feature (a key, a notch, an asymmetric corner) that only fits one way. The conveyor and station recognize it.
- Orientation sensor: A sensor checks the pallet’s orientation before it locates. If it’s flipped, the system faults (or rotates the pallet).
- Conveyor guides: The conveyor’s side rails only allow the pallet through one way (a asymmetric guide block).
The pallet that came back rotated had no orientation feature. It was symmetric. Add a key (or an asymmetric corner) so it can only travel one way.
The pallet rule: Two pins (round + diamond) with replaceable bushings. Make the pallet non-symmetric (an orientation key) so it always arrives the same way. The pallet that wouldn’t locate was symmetric — no orientation feature. Add the key.
Step 3: Palst Stop and Lift
At each station, the pallet stops and lifts off the conveyor (so it doesn’t ride on the conveyor belt during work).
- Stop (stopper): A pneumatic stop (a blocker) raises and stops the pallet at the station.
- Lift (lift table): A lift mechanism raises the pallet (with the part) off the conveyor. The pins engage the bushings. The pallet is now rigidly located (not on the conveyor).
- Lower: After the work, the lift lowers the pallet back onto the conveyor. The stop retracts. The pallet moves on.
The lift ensures the pallet is rigid (not on the flexing conveyor belt). The pins locate it. The work (assembly, tightening) happens on a rigid pallet.
Step 4: Part Location on the Pallet
The part sits on the pallet. It needs its own locators (article 69) on the pallet:
- 3-2-1 locators: The part sits on three pads, two side locators, one stop.
- Clamps: The pallet clamps the part (or the station’s clamps act on the pallet).
- Quick-change: For multiple part types, the pallet has quick-change fixturing (pins, locators) that swap.
The part’s datum on the pallet matches the station’s datum. Every station sees the part at the same position (relative to the pallet).
Pallet Material and Construction
- Aluminum plate: Light, easy to machine. Standard for pallets.
- Steel plate: Heavier, more rigid. For heavy parts or precision.
- Cast aluminum: For high volume (the pallet is a casting). Lower cost at volume.
The pallet needs hardened bushings (for the locating pins) and wear strips (where it rides on the conveyor).
Conveyor Interface
The pallet rides on the conveyor. The conveyor type depends on the pallet:
- Roller conveyor: The pallet slides on rollers. Standard for circulation.
- Chain conveyor: The pallet is carried on chains. For heavy pallets.
- Belt conveyor: Less common for pallets (the belt wears).
The pallet’s bottom has wear strips (UHMW plastic or steel) that slide on the rollers. Replaceable when worn.
| Element | Design |
|---|---|
| Pallet body | Aluminum or steel plate, machined flat |
| Locating bushings | Hardened, replaceable |
| Orientation feature | Key, notch, asymmetric corner |
| Wear strips | UHMW or steel, replaceable |
| Part fixturing | 3-2-1 locators, clamps |
A Pallet Design Checklist
- What is the part? (Size, weight.)
- What is the pallet material? (Aluminum, steel?)
- Locating: round pin + diamond pin with bushings?
- Are the bushings replaceable?
- Is the pallet non-symmetric? (Orientation key?)
- How does the part locate on the pallet? (3-2-1?)
- Are there clamps? (On the pallet or station?)
- Wear strips on the bottom? (Replaceable?)
- Stop and lift at each station? (Pneumatic?)
- Orientation sensor? (Detects flipped pallet?)
- Quick-change fixturing? (For multiple parts?)
- Pallet weight? (Conveyor capacity?)
The Bottom Line
Pallet and fixture design for circulation is repeatable location and orientation. The pallet that wouldn’t locate on the return was symmetric — no orientation feature. Use two pins (round + diamond) with replaceable bushings, make the pallet non-symmetric (a key), and add a stop-and-lift at each station. The pallet that locates every time wasn’t the heaviest one — it had orientation and replaceable bushings.
