A pulley or coupling is fitted to a shaft with a parallel key. It works initially, then develops play, the keyway elongates, and the connection slips under load. The shaft and hub are strong enough. The key is the weak and the wearing element, and a key that is fitted loosely or sized wrong ruins both the shaft and the hub long before either is overloaded.
What the key does
A parallel key sits half in a keyway machined in the shaft and half in a keyway in the hub. It transmits torque by locking the hub to the shaft, carrying the load through the sides of the key. The top of a parallel key normally has a small clearance; the driving contact is on the flanks, not the top and bottom. This is why a key can be the correct height but still fail if the side fit is loose.
Keys are cheap and standardized, selected from shaft diameter. They are deliberately made replaceable so that an overload damages the key rather than the shaft, provided the rest is designed correctly.
Two failure modes: crushing and shear
The key transmits torque through a force on its side. That force can shear the key across its cross-section, or it can crush the key or the keyway surface by excessive bearing pressure. Crushing or permanent deformation of the flank usually appears first on a properly proportioned key, elongating the keyway and producing play. Shear splits the key into two pieces in a sudden overload.
The torque capacity from crushing uses the contact area, key length times half the key height, times allowable bearing pressure. Shear capacity uses key width times length times allowable shear stress. Both are checked, with the crushing condition often governing.
The fit matters as much as the size
A key that is a sliding fit with clearance rattles under reversing and shock loads. Every load reversal hammers the flank, and the keyway wears larger over time. For drives that reverse or see impact, the key should fit the sides closely rather than drop in loosely. A new key in an already worn keyway does not restore the fit; the elongated slot must be repaired or a new keyway and key made.
Hand-fitting a key to full, even contact along its length distributes the load. A key touching only at the ends concentrates the pressure and locally deforms the keyway even when the total length looks adequate.
Length and the hub
The effective key length is limited by the hub length and, for a standard circular-end keyway, by the end that cannot carry full load. Don’t assume the full hub length is effective. A short hub with a small key is a common bottleneck on components selected for bore rather than torque. If the hub cannot be longer, use a larger or stronger key, or splines for high torque.
Set screws and interference
A set screw over the key helps locate the hub axially and adds clamping, but it is not the torque drive; relying on the set screw alone marks the shaft and slips. Interference fits and clamping hubs transmit torque without a key or alongside one and can handle reversing loads better, but need controlled assembly. Don’t let a set screw substitute for a properly fitted key in a high-torque drive.
Reversing and shock duty
Connections on reversing machines, crushers, and punch drives see the key loaded first one way then the other, which is exactly where loose keys and worn keyways develop. Close fits, adequate length, and sometimes two keys or splines suit these duties. A key sized for steady one-direction torque may be inadequate under repeated reversal even at the same nominal load.
Splines for higher torque
Where a single key cannot carry the torque within the shaft size, splined connections spread the load over several teeth, reduce contact pressure, and allow axial movement. They cost more to machine. Multiple keys are a middle option. Don’t keep enlarging a single key beyond the shaft proportions; the keyway itself weakens the shaft if too wide.
Repair considerations
A worn shaft keyway can be welded and remachined, or a larger keyway cut with a matching key, while preserving shaft strength. A worn hub keyway often means replacing the hub or bushing. Don’t fit an oversize key into an elongated slot without machining; it concentrates load and hides the loss of original fit. Check shaft strength after any keyway enlargement.
A worked torque check
Take a 50 mm shaft driving 500 Nm through a 14 by 9 mm key in a 70 mm hub. The force at the shaft surface is torque over radius, about 20 kN. The bearing area on one flank is the effective length, say 60 mm, times half the key height, 4.5 mm, giving 270 square millimeters and a pressure around 74 MPa. If that exceeds the allowable pressure for the key and shaft material, the keyway deforms; lengthening the hub or using a wider key lowers it. The same force across the shear area, 14 by 60 mm, gives a shear stress near 24 MPa, usually well within limits, confirming that crushing governs before shear in most steady drives.
Key types and when to use them
Parallel square or rectangular keys are the general standard for shafts and hubs. Woodruff keys are small, semicircular keys that self-align and suit light drives and machine tools. Tapered keys lock tightly when driven in for heavier or reversing duty but need careful fitting. Gib-head keys allow removal. Select the type from load and assembly rather than using a generic parallel key everywhere; a tapered fit is often the answer where a parallel key keeps coming loose.
Effect of keyways on shaft strength
A keyway cuts into the shaft and creates a stress concentration at its corners, reducing fatigue strength, especially at steps where bending is high. High-speed and heavily bent shafts need the keyway placed and radiused carefully, and the shaft diameter may need to increase to restore margin. Don’t size a shaft for bending and torsion and then ignore the weakening from a wide, sharp-cornered keyway; the keyway is often the actual fatigue origin.
Keyless connections
Shrink fits, clamping bushes, and friction-locking assemblies transmit torque without a keyway and avoid its stress concentration and backlash. They suit reversing and precision drives and simplify balancing, but require controlled heating, hydraulic fitting, or torque procedures. For high-speed spindles and servo axes, a keyless clamped connection is often preferable to a keyed one. Compare its assembly requirements against the maintenance setting before choosing.
Balancing and high speed
At high rotational speed, an exposed key and keyway upset balance. Use a matching taper or fill key so the assembly balances, and consider the key as part of the rotor balance rather than adding it afterward. Pulleys and couplings supplied with an open keyway need the correct fitted key to balance; a missing or partial key causes vibration that is then wrongly attributed to the bearings.
Assembly practice
Clean and deburr the keyways, fit the key to the shaft first, and slide the hub on without forcing. Check that the hub seats fully and the key does not ride on the top clearance. Mark and record the key size and fit. Drifting a hub over a tight key can roll the edges and create the very looseness being avoided. After assembly, confirm torque transfer and check for fretting at the joint during service.
Diagnosing key failures
Play between shaft and hub, a rocking pulley, and elongated polished keyway surfaces indicate a loose, crushing key. A sheared key in two pieces points to a sudden overload or jam. Fretting rust around the keyway shows micromovement under load. Identify whether the root cause is an undersized key, a poor fit, or an abnormal overload before fitting a replacement; replacing with the same loose key repeats the wear.
Common mistakes
Using the top of the key to drive instead of the flanks, leaving side clearance on reversing drives, fitting a new key into a worn slot, and ignoring shaft weakening from the keyway are the recurring errors. Treating the key as permanent rather than a sacrificial, replaceable element leads to damaged shafts. A correctly fitted key is a controlled fuse; a loose key is a hammer that enlarges the slot.
Taking a few minutes to fit the key to full side contact at assembly, rather than dropping in the nearest standard part, prevents the slow wear that turns a cheap component into a damaged shaft and hub.
Taking a few minutes to fit the key to full side contact at initial assembly, rather than dropping in the nearest standard part, prevents the slow wear that turns a cheap component into a damaged shaft and hub.
Bottom line
A parallel key drives through its flanks and fails by crushing the keyway or shearing, with close side fit and full contact essential under reversing and shock loads. Size width and effective length for the torque, don’t rely on set screws, and use splines where one key cannot fit within the shaft. Repair worn keyways properly. Most key failures are fit and wear failures rather than simple overloads.
