Many mechanical and motorized devices include mechanical rotary drive shafts or torque transmission components that are used to transmit torque and rotation between adjacent or inline components of a drive train. These motors, shafts, couplings and drive line components are potentially subjected to torque loads that are higher than the intended normal or maximum operating torque for a given purpose. These higher torque loads can come from rapid acceleration or deceleration events or an internal source such as a decrease of mechanical efficiency downstream due to contamination, wear, or misalignment. The torque increase may also come from an external overload or an improper or failed connection of the drive components. Torque overload can exceed the safe design limits of the device and cause damage and/or reduce the lifespan of the drive components and cause the device to malfunction.
Conventional wrap springs have traditionally been used to protect against torsional overload in a singular direction when a drive source, like a motor or manual input drive device, produces too much torque due to a gradual or sudden increase of drive torque resistance. A conventional unidirectional wrap spring is basically a helical spring with an input shaft (which is connected to a drive source) interference fitted into one end of the spring, and an output shaft (which is connected to a driven component) interference fitted into the opposite end of the spring. The outer diameters of the shafts and the inner diameter of the spring are specified and controlled to provide a specified interference fit when assembled. When the input shaft is torqued, the friction along the interference fit causes the coils to reduce the grip, and when the torque value increases to a balanced friction fit, the wrap spring releases the input shaft (i.e., allows the input shaft to torsionally “slip”). For example, assuming the wrap spring is a right hand helix and the input shaft rotates in a clockwise direction, when the torque from the input shaft exceeds the threshold amount in the clockwise direction, the helix feature is torqued in the unwind direction causing the wrapped coils to expand and release the input shaft, and thus avoid torsion overload of the drive shaft system. As this input torque is reduced, the spring forces once again clamp down to the original condition. The unidirectional wrap spring only releases when the torque exceeds the predetermined amount in a single direction, e.g., the clockwise direction.
Conventional bidirectional wrap spring designs exist that protect against torsional overload in both rotational directions. They differ from unidirectional wrap springs in that the opposite end of the spring is interference fitted along the outside diameter into a cylindrical feature added to the output shaft. Thus, when the input shaft reaches a threshold torque in the counter-clockwise direction, the wrap spring holds tightly to the input shaft but reduces the compressive radial friction grip with respect to the inner diameter of the output shaft cylindrical feature. This allows slip between the outside of the spring and the output shaft. These conventional bidirectional wrap springs, however, require machining inside and outside component diameters to have very close tolerances to provide the required interference fits. It is difficult to machine the parts such that the interference fit torsional slipping is balanced in both rotational directions. Bidirectional wrap springs also require more complicated componentry than unidirectional wrap springs. Moreover, the coupling connection between the output shaft cylindrical feature and the spring in a bidirectional wrap spring takes up more space and weighs more than the connection between the output shaft and the spring in a unidirectional wrap spring.