I was splitting wood one day a few years back, getting next winter’s firewood ready on my hydraulic splitter. It normally handled my ash and oak with ease, but I had a particularly gnarly piece of birch queued up, and the splitter was struggling. The 20-ton cylinder slowed as the wedge jammed in the twisted grain, the engine started to bog down, then BANG! I jumped back as something gave way and the engine revved out of control; I figured a hydraulic hose gave out. Whatever it was, I was done for the day.
I later discovered that a coupler between the engine shaft and the hydraulic pump failed dramatically. It was an easy fix once I ordered the right part, and I’ve since learned to keep extras in stock. Couplings are useful things, and they’re the next up in our series on mechanisms.
Matching the Mismatched
Couplings like the one that failed on me have a few general uses. First and foremost, they do exactly what the name implies: they couple together two shafts to transmit rotational force. Most couplings transmit rotation between shafts that are more or less on the same axis, with some exceptions that we’ll cover below.
Depending on the needs of the design, couplings can either be rigid or flexible. Rigid couplings are for well-constrained systems where the two shafts are not likely to get out of perfect axial alignment. Rigid couplings include sleeve or muff couplings which generally clamp to the shafts with grub screws and which may have a keyway machined into the bore for better torque transmission.
If there’s any chance of misalignment between shafts, though, a flexible coupling is called for. While not designed for transmitting torque through a wide angle, flexible couplings do allow shafts to get as much as a few degrees out of alignment without causing damage. This is important in devices where vibration or flexibility in the structure is likely to let the shafts move slightly relative to each other, like on my log splitter. In that case, the shafts were coupled by a jaw coupling, often known by the trade name Lovejoy. Lovejoy couplings have two metal halves with castellations that nest within each other. The metal is prevented from contacting directly by a tough elastomeric spider which both cushions rapid changes in torque and allows the shafts to flex a bit.
Another flexible coupling often seen in hardware projects is the beam coupling. Commonly found between stepper motors and lead screws in larger CNC machines, beam couplings look a little like stocky springs that have been machined out of a single cylinder of material. The springiness allows for a little angular flexibility and even for some end play which could allow the shafts to get closer or farther apart. Beam couplings are typically machined from aluminum, but other metals and even polymers are used too.
Joints: Universal and Constant
When misalignment between shafts is more than a couple of degrees, or when a lot of torque needs to be transmitted through a wide range of angles, different kinds of couplings are called for. The canonical coupling in this category would probably be the universal joint, commonly used in automotive driveshafts. The familiar design of nested yokes joined by a cross or spider dates back far into history, with early examples showing up on Greek siege engines. The great English scientist Robert Hooke would later study these devices and characterize them mathematically. He realized that the output rotation was not necessarily constant at every angle and worked out how to fix this: a pair of joints at each end of a shaft rotated 90° axially. This arrangement can still be seen on conventional driveshafts for rear-wheel-drive vehicles, with a U-joint at the output of the transmission and another at the differential input.
For shorter shafts, like on the drive axles of front-wheel-drive vehicles, there may not be room for a pair of U-joints. To accommodate this, another flexible coupling is used: constant velocity (CV) joints. CV joints look a little like beefy ball bearings with concentric races with deep, wide grooves. Large steel balls ride in each groove and transmit torque between the inner and outer races. CV joints can work through a large angular range and are usually used in pairs on each axle.
There is a wide range of other couplings that we haven’t touched on here, from flexible couplings that use metallic bellows or even fabric or polymer diaphragms to couplings like torque converters and magnetic couplings that allow for rotational slippage. For almost every application requiring torque to be transmitted between shafts, there’s a coupling that’s right for the job.