Introduction to Torque Bushings & Couplings Principles
Expanding Bushings and Squeeze Couplings from Power Transmission Engineering, Inc. offer a cost-effective, off-the-shelf, alternative to QD style tapered bushings, splines, keys, or press fits for mounting / securing gears, timing pulleys, sprockets, conveyor pulleys, or couplings to shafts.
A keyed connection has been assumed to be the low-cost method of joining a part to a shaft. But, when one considers the cost and skill required to accurately mill the keyway in both components and fit the key, a keyless alternative is cost effective. Our SqueezeTorqueCouplingsJ (STC) and ExpandingTorqueBushingsJ (ETB) are a cost-effective alternative, and proven means of connecting components without keys and setscrews. Both the ETB Series and the STC Series provide the torque capacity and longevity of interference (shrink) fits, with convenience of a clearance fit, easy assembly, with the capability for infinite repositioning and reusability.
The Keyless Advantage
Reliably transmit high
torques, thrust loads and bending moments.
$ Less assembly time and expense than keyed and splined connections.
$ No backlash under bending moments, shock or reversing loads.
$ Reusability, unlike most competitive bushing systems.
$ Greater convenience than press-fits, no lengthy calculations, hot oil, bulky presses or jigs.
$ No stress concentration at keyway, allowing a smaller shaft diameter, for a given torque.
The Problem with Keys
Most equipment stops, starts and is subject to vibration. This causes shock to resonate at the keyed connections. Over time, the keyed connection to the hub begins to move and rub on each other. This movement causes a wear phenomenon known as fretting corrosion.
Keyway wear increases the movement and backlash between the key, shaft and hub. These impact loads result in a concentration of force at the key, often resulting in the shaft cracking at the keyway. Impact loads can also pound out the keyway over time, resulting in the key shearing or rolling, causing the connection to fail.
Often a repair can be made, using an Expanding Bushing over the existing cracked shaft
Both the ExpansionTorqueBushingJ (ETB) and SqueezeTorqueCouplingJ (STC) use the principle of the inclined plane, also known as the wedge principle, with compounded force from the lever principle. Clamp force is created by tightening several screws, located in a radial position around the tapered collar. As the screws are tightened, the wedges are drawn together. The multiple screws combined force is comparable to a lever arms compounded force, see Fig. 2.
This axial force draws the two tapered wedges together, which changes the force to radial. This mechanically eliminates the gap between the wedges, shaft and hub. The wedges= radial force is uniformly applied against the hub and shaft, mechanically transmitting both torque and thrust loads. This wedging action provides a tremendous amount of mechanical force in a compact design that is reliable, easy to use, disassemble and is reusable.
The force / clamp load can be increased in two ways. First, by having two tapers that work in tandem as they are drawn together. Second, by increasing the number of screws, which increases the mechanical clamp load by a linear function.