Quick answer: pick the sprocket after the belt, never before. The belt series fixes the tooth form and the pitch, tooth count then sets the drive pitch diameter and how much the belt speed swings on every revolution, and the bore decides how torque gets from the shaft into the plastic. On each shaft only one sprocket is locked axially, normally the centre one, and every other sprocket is left free to slide sideways so the belt can expand.
Decide in this order
Belt series first. Then tooth count. Then bore. Then how many sprockets sit across the width. Any other order ends with a machined shaft and no sprocket that fits it.
Sprockets are not interchangeable between series even when the pitch matches. A 976 belt and a 986 belt both run on a 25.4 mm pitch, and the sprockets are still different parts because the tooth has to sit in a different rod pocket. Start from the series and browse the matching part on the sprockets and drive wheels page rather than from a pitch number alone. If you have not fixed the belt yet, the belt selection guide comes first.
What tooth count actually changes
A plastic modular belt does not wrap a sprocket as a circle. It wraps it as a polygon with one flat per tooth. The driving radius therefore rises and falls between the tooth tip and the chord across the flat, once per tooth, and belt speed rises and falls with it. This is the polygon effect, and it is the single reason tooth count matters beyond fitting the frame.
Fewer teeth means a smaller sprocket, a shorter nose, and more speed variation. More teeth means a smoother belt and a bigger drive package. The numbers below are calculated from the catalogue pitch of the 900 series (27.2 mm), not taken from a datasheet, but the shape of the trend holds for every series.
| 900 series teeth | Pitch diameter (calculated) | Speed swing per tooth | Where it belongs |
|---|---|---|---|
| 9 | about 79.5 mm | about 6% | Tight transfer noses, short centres, low speed |
| 12 | about 105 mm | about 3.4% | General transport, the usual first choice |
| 18 | about 157 mm | about 1.5% | Filling, weighing, vision inspection, high speed |
| 20 | about 174 mm | about 1.2% | Long heavy runs, smoothest option in the series |
Read that table as a trade, not as a ranking. A 6% speed swing is invisible on a crate line running at 12 m/min and it will visibly shake liquid in an open container under a filler. Low tooth counts exist so you can get a small transfer nose, and that is what they should be used for. If unstable products are the problem, the fix is a high tooth count at the drive plus a proper transfer at the ends, not a stiffer belt.
One more thing the tooth count does: it changes torque. Doubling the pitch diameter doubles the torque the gearbox has to give for the same belt pull. Fix the tooth count before the drive is sized, or the gearbox comes back undersized.
Tooth counts available by series
These are the counts we stock as standard. Series not listed here are built to the tooth count the machine needs rather than from a fixed list, so ask before you design around a number.
| Series | Belt pitch | Tooth options | Sprocket page |
|---|---|---|---|
| 300 radius | 46 mm | 12 | 300 sprockets |
| 400 | 50.8 mm | 10 / 12 / 16 | 400 sprockets |
| 900 | 27.2 mm | 9 / 12 / 18 / 20 | 900 sprockets |
| 956 | 50.8 mm | 8 / 10 / 12 | 956 sprockets |
| 976 | 25.4 mm | 12 / 16 / 20 | 976 sprockets |
| 986 | 25.4 mm | 12 / 16 / 20 | 986 sprockets |
| 1000 | 25.4 mm | 16 / 18 / 22 | 1000 sprockets |
| 1100 | 15.24 mm | 16 / 24 / 32 | 1100 sprockets |
| 2400 radius | 25.4 mm | 16 | 2400 sprockets |
| 4809 | 57.15 mm | 9 / 14 | 4809 sprockets |
| 5935 | 19.05 mm | 9 / 13 / 17 / 24 | 5935 sprockets |
| 7705 | 25.4 mm | 10 | 7705 sprockets |
| OPB | 50.8 mm | 8 / 10 / 12 | OPB sprockets |
Note the pattern in the fine pitch series. The 1100 belt runs on a 15.24 mm pitch, so even a 32 tooth sprocket stays a compact wheel. Fine pitch is how you get a smooth drive and a small diameter at the same time.
Bore: square, round, or bushing insert
All three bore styles run in POM (acetal). What differs is how torque crosses from the shaft into the sprocket and how much freedom you keep during installation.
| Bore type | Sizes | How it drives | Use it when |
|---|---|---|---|
| Square bore | 40 / 50 / 60 mm | The flats carry torque directly, no key, no grub screw | Driven shafts, wide belts with several sprockets on one shaft, washdown |
| Round bore | 25 / 30 / 40 mm | Needs a key or a clamp to transmit torque | Idler and return positions, or an existing round shaft you cannot change |
| Round bore plus bushing insert | Inserts 30 / 40 / 50 / 60 mm | Insert fills the bore down to the shaft you actually have | Retrofits and mixed shaft sizes, so one sprocket body covers several machines |
For a new build, use a square shaft on the drive. It is the cheapest reliable way to turn six or eight sprockets from one shaft without keyways, and there is nothing to loosen in a wet environment. Round bore belongs on idlers, or on a retrofit where the shaft is already in the machine. The round bore bushing inserts exist for exactly that second case: the sprocket bore is standard and the insert adapts it down to the shaft diameter on site.
One sprocket fixed, the rest floating
This is the rule people skip, and it is the one that cracks sprockets. A plastic belt grows and shrinks with temperature and with water uptake. Across a 1200 mm wide belt the change is real, and it is measured in millimetres, not microns. Our note on thermal expansion in plastic modular belts covers how much to allow.
If every sprocket on the shaft is clamped, the belt has nowhere to go. It buckles, the teeth climb, and the edge modules split. The standard layout is one sprocket locked axially at the centre of the shaft using fixed bearing bushings, and every other sprocket free to slide along the shaft. The belt then grows outward in both directions from the centre and the sprockets follow it.
Two details make this work in practice. The floating sprockets must be free, not lightly nipped, so check them by hand after the guards go on. And the centre sprocket must be genuinely centred, because that is the point the belt tracks from.
How many sprockets across the width
A rough working figure for general transport is one sprocket every 100 mm to 150 mm of belt width, always an odd number so one of them can sit on the centreline. A 600 mm belt normally takes five, a 1000 mm belt takes seven or nine.
Tighten that spacing when the load per square metre is high, when the belt is a heavy coarse pitch such as the 4809 raised rib belt at 57.15 mm pitch, or when product is dropped onto the belt near the drive. Open it up when the belt is fine pitch and lightly loaded, for example a 1100 flat top belt carrying small packs.
Match the idler and return shafts to the drive. Sprocket count and spacing should be the same at both ends, otherwise the belt is supported differently at each end and it will wander off line.
What to send with the enquiry
- Three numbers, in this order: belt series, tooth count, bore type and size. That alone is a quotable sprocket.
- Shaft form and dimension as it is today, square or round, plus the shaft length between bearings.
- Belt width and the number of sprocket positions you have room for.
- Line speed and what is being carried, so we can tell you whether the tooth count you picked will shake the product.
- Working temperature and whether the line is washed down, which affects how much float to leave.
- If it is a retrofit, one photo of the shaft end and one of the old sprocket beside a tape measure, since keeping the existing sprockets depends on how far the teeth have worn.
Send that to the contact form or by WhatsApp and we will come back with the sprocket part, the bore option that suits your shaft, and how many to put across the width.
