When changing pulleys on a shaft already in line, the QD bushing size chart prevents both slipping under load and getting it off easy once you’re done. It shows you what series will fit your shaft, what kind of torque the bolts can withstand, and how they work at various duty levels based off the taper.
They group their bushings by size into a dozen series running from tiny JA to massive P. Within each series, there is outside diameter, bore range, length through bore, keyway size, and suggested bolt torque. This progressive design serves a purpose. The smaller sizes keeps the package light enough for modest loads. However, the larger sizes adds more bolt capacity and wall thickness so the grip holds hundreds of foot-pounds without walking on shaft.
How to Use the QD Bushing Size Chart
As series increases in size, torque increases consistently, not because of marketing but because of mechanics. A JA bushing at 20 ft-lb is right for driving light fans. That identical pulley on a crusher require the 200-plus ft-lb range of an N or P. Instead of guessing whether your bolts are strong enough, you can use the chart; additionally, the torque values rises as the mechanical demands of the application increase.
When installing, you slip the bushing onto the shaft, align the keyways, let the tapers seat together and back the bolts down in a star pattern. The taper turns axial bolt force into radial clamping pressure, locking the bushing without crushing the shaft. To remove it, reverse the process by moving your bolts to the threaded jack holes. This lets the taper release easy, allowing you to change out these bushings repeatedly.
It’s as simple as looking at the chart in order and measuring the shaft. Then, pick the first series where the bore range include the measurement. Finally, verify that it has a torque rating higher then the one generated by your drive. Where two series match bore sizes, pick either one with the longer hub (offering greater support) or the next larger torque value.
Most QD bushings is made of cast iron, which holds compression well and machines cleanly. However, there is something else to consider. In wash-down environments or for shock loads, you’ll want to use stainless steel, but you should of still stay within the same series numbers from the chart.
The chart also keeps me from forcing a marginal fit. You know you have one on with a one series smaller bushing because it clamps initially. Then, the bolts will grips toward the bottom end of their rating, which loosen the hub over time. There’s not much more cost for a bit larger size up, though; that extra margin pays off in spades when you get a jam further down line. That margin turns a routine pulley change into a five-minute job instead of an afternoon of frustation.
