Quick answer: guide strips are moulded ribs on the underside of a modular belt that run inside a slot machined along the conveyor frame. They hold the belt on line without side wear strips pressing on the belt edges. A belt can be flat underneath, or carry a single strip, or two strips. Fit a flat-bottom belt into a frame built for guides and it will drift sideways and start climbing off the sprockets within one shift, because nothing is left to locate it. Turn a module over and photograph the back before you order anything.
What the strips are actually doing
A modular belt is driven positively by sprockets, so nobody needs to tension it. What the sprockets do not do is keep the belt centred across the frame. Load sitting off centre, an out of square shaft, a wet return run, any of these push a belt sideways, and once the edge finds the frame it wears fast.
Two solutions exist. Either you line both sides of the frame with wear strips and let the belt edges rub against them, or you put a rib on the underside and a slot in the frame and let the two engage. The second is quieter, wears far less, and takes load off the belt edge where the modules are weakest.
The cost is that belt and frame now have to agree. A guided belt is not a better belt, it is a belt matched to a particular frame, and the match is not visible from the top surface.
What you see underneath, and what it means
Get a module free, or reach the return run underneath the conveyor, and look at the back. These patterns cover almost everything.
| What you see underneath | What it means | What to measure |
|---|---|---|
| Flat bottom, only the hinge barrels standing proud | The belt is tracked by side wear strips, or by the sprockets alone on a short conveyor. No frame slot needed. | Nothing extra. Record that the underside is flat, so we do not quote a guided mould. |
| One continuous rib running along the belt, in line with travel | Single guide strip. There is a matching slot in the frame, usually near the centre. | Strip width and height, distance from the belt edge to the strip centre, and the width of the frame slot. |
| Two ribs running along the belt, spaced apart | Double guide strip. Used on wider belts and heavier duty, where one rib cannot take the side load. | Both of the above, plus the centre to centre distance between the two strips. |
| Ribs across the belt rather than along it | Not guide strips. That is a raised rib surface seen upside down, or a flight base. | Photograph the top surface as well so we can tell the two apart. |
| A frame with a machined slot but a flat belt in it | Somebody has already replaced this belt with the wrong mould. Expect tracking complaints. | Slot width, slot depth, and slot position across the frame. |
The last row is common enough to name. A plant orders a replacement by pitch and surface, the belt arrives flat underneath, maintenance fits it because it goes on the sprockets, and the line runs crooked from then on. Nobody connects a belt that will not track straight back to a purchasing decision made months earlier.
Why a flat-bottom belt drifts, and how fast
Without a rib in a slot, the only things resisting sideways movement are the sprocket teeth in the module pockets and whatever friction the carryway provides. Neither is a locating feature. Sprocket teeth engage the drive bars in the direction of travel; they let the belt slide along the shaft, which is deliberate, because only the centre sprocket is locked and the rest float, so the belt is free to grow and shrink with temperature.
So a flat belt in a guided frame does not fail dramatically. It creeps. One edge finds the frame, starts to wear and shed material, the belt goes out of square, and the modules at the worn edge take load they were never meant to take. On a three shift line you see the difference in a day and hear it in a week.
The reverse error is less common but worse. Fit a guided belt into a frame with no slot and the rib rides on the carryway. Now the belt runs on two narrow lines of plastic instead of its full underside, contact pressure goes up by an order of magnitude, and the strips are ground away in weeks.
Why the strips sit grouped on one side
On a wide belt you often see the guide strips grouped toward one side rather than spread symmetrically. That looks like a mistake and it is not.
Plastic belts change length with temperature, and width as well. A belt going into a freezing tunnel at one end and a wash at the other moves a measurable amount across its width, and that movement has to go somewhere. Anchoring it at two widely separated points would fight the movement and load the modules in a direction they are not designed for. Grouping the strips locates the belt on one side and leaves it free to grow toward the other. The calculation behind the allowance is in our note on thermal expansion in plastic modular belts.
The practical consequence for a replacement: record which side the strips are on, looking along the direction of travel. A guided belt built as a mirror image of the one you have will not drop into the frame.
What else changes when the underside changes
The strips sit in the belt path, so anything the belt path touches has to clear them. Three items come up regularly.
| Component | Why the underside matters |
|---|---|
| Return wheels | Centre block and edge block wheels have to be positioned so they do not sit under a strip. Moving a wheel is cheap; discovering the clash on site is not. |
| Pressure rollers | A hold down roller on an incline presses on the belt back. It has to bear on the module, not on the rib. |
| Side guards | If the belt is guided, side guards are containing product rather than tracking the belt. That changes the height you need. |
Module thickness comes into the same clearance sum. A 1000 series flat top is 8.7 mm thick, a 900 series flat top is 10 mm and a 966 raised rib is 14.5 mm. Add a guide strip on the underside and the total stack against the frame changes again, so measure the frame opening rather than assuming the new belt sits where the old one did.
What to record before you order
Four items settle the underside completely. None of them needs the line to be stripped down, and none of them can be recovered from the belt code on the old order, because the underside is the field that goes missing in transcription.
- Strips present or not. A yes or no, from a photograph of the back of a module.
- Single or double. Count the ribs running in the direction of travel.
- Position across the belt. Distance from one belt edge to the centre of each strip, and which edge you measured from, looking along the direction of travel.
- Frame slot width and depth. Measure the slot in the conveyor, not the strip on the worn belt. A strip that has run for years is smaller than it started.
That last point is the one worth repeating. Guide strips wear, and they wear on the flanks where they rub the slot. Measuring the old strip and ordering to that number gives you a belt that is loose from day one. Measure the slot, and let us size the strip to it.
Send this and we can name the mould
One photo of the underside of a module, one of the top, the four items above, plus the pitch and the belt width. If you have not measured pitch yet, the method is in our guide on measuring a modular belt for replacement, and the rest of the range is listed under straight running modular belts.
Photos of an underside travel better through WhatsApp than through email, since the return run is usually dark and phone images from email get compressed. Lay a ruler across the strips before you shoot. You can also send everything through the contact form, and we will tell you which of the four items is still missing rather than quote around it.
