Pitch scales, the item does not
Pitch is a distance the belt travels between one item and the next, so it scales exactly with the speed ratio. The item is a physical object and scales not at all. The gap is what is left over, which makes it far more sensitive to the ratio than the pitch is.
At the defaults: a 24 in item with a 6 in gap has a 30 in pitch on the infeed. Double the belt speed and the pitch becomes 60 in, the item is still 24 in, and the gap is 36 in. The speed went up 100 percent, the pitch went up 100 percent, and the gap went up 500 percent. That leverage is the reason gapping belts work at modest speed steps.
The rate is unchanged, and this matters
60 ft per minute on a 30 in pitch is 24 items a minute. 120 ft per minute on a 60 in pitch is also 24 items a minute. The ratio appears on the top of the rate expression and inside the pitch on the bottom, so it cancels, and it would cancel for any ratio at all. Items are not created at a transfer.
A gapping belt therefore buys space and never throughput. Lines get one fitted to fix a rate shortfall and the shortfall stays exactly where it was, because it was somewhere upstream all along.
Negative gap means a queue
Slow the outfeed below the ratio of item length to infeed pitch, which is 24 over 30 or 0.8 here, and the arithmetic asks for a pitch shorter than the item. That does not happen physically. What happens is that product backs up at the transfer, the gaps on the infeed close as it does, and the line becomes an accumulator. The negative number is the page telling you the belt is no longer the thing setting the spacing.
Two constraints, not one
If a downstream scanner wants a 48 in gap, the ratio has to be 72 over 30, which is 2.4, so the outfeed runs at 144 ft per minute. But there is a second reading of the same requirement: at whatever speed the outfeed actually runs, a 48 in gap means one item every 72 in of belt, which at 120 ft per minute is 20 items a minute. If 24 are arriving, the gap requirement and the arrival rate are in conflict and something has to give — accumulation upstream, metering at the infeed, or a faster outfeed. Checking only the ratio and not the rate ceiling is how that conflict gets discovered during commissioning.
The transient
None of this describes the moment of transfer itself, when the item is on both belts at once for its own length and being pulled by two surfaces at different speeds. That is where product skews, slips and tips, and it is worse for tall, light or short items. The geometry of the transfer is a supplier question with a real sample on the bench, not an arithmetic one.
Questions people ask
How do I calculate the gap between items after a speed change?
Multiply the infeed pitch by the speed ratio to get the outfeed pitch, then subtract the item length. A 24 in item with a 6 in gap has a 30 in pitch; at double speed the pitch is 60 in and the gap is 60 minus 24, which is 36 in. The item length does not scale, which is why the gap grows by much more than the ratio.
Does a faster conveyor increase throughput?
Not at a transfer, no. The same items arrive and the same items leave, so the rate is identical either side. Speeding the outfeed increases the pitch and opens the gap, which is a spacing change, not a capacity change. If a line is short of rate, the constraint is upstream of the transfer and a faster belt will not reach it.
What speed ratio do I need for a 48 inch gap?
Add the gap to the item length to get the pitch you want, then divide by the pitch you have. A 48 in gap on a 24 in item wants a 72 in pitch, and against a 30 in infeed pitch that is a ratio of 2.4. From a 60 ft per minute infeed the outfeed runs at 144 ft per minute.
What happens if the outfeed is slower than the infeed?
The gap closes. Below the ratio of item length to infeed pitch the arithmetic asks for a pitch shorter than the item, which cannot happen, so product queues at the transfer instead. The line starts behaving like an accumulation conveyor whether it was designed as one or not, and the negative gap on the page is the signal that the belt has stopped controlling the spacing.
Why did my gapping belt not fix the line rate?
Because a speed ratio cannot change a rate. It moves items further apart while sending exactly as many of them per minute as it receives. Whatever is limiting the line is upstream of the transfer, and the honest next step is to measure the arrival rate at the infeed rather than adjusting the outfeed speed further.