Shaft couplings

You need the torque (from power and speed) multiplied by a service factor for the machine type, both shaft diameters, and the gap between shaft ends. The service factor rises for shock loads such as crushers or reciprocating compressors. The coupling must also accept the actual misalignment of the machine train.

Jaw couplings with an elastomer spider are compact, damp vibration and allow easy spider replacement, suiting small to medium pumps and fans. Tyre couplings accept large misalignment and absorb shock well. Disc couplings need no lubrication and have no backlash, suiting precise or high speed drives.

Common causes are misalignment beyond limits, hub faces set too close or too far apart, or a spider material unsuited to the heat and oil at the location. Spiders come in several hardnesses, often identified by colour, and changing hardness changes the torque rating and damping. Check alignment with a dial indicator or laser after installation.

Many jaw coupling designs share common dimensions, so spiders can sometimes be interchanged, but check the number of jaws, outside diameter and spider thickness. Tyre and disc couplings differ in assembly dimensions between makers, so the complete set is usually replaced. Note the existing coupling code and measured dimensions.

Each coupling type has a specified gap between halves, often listed as dimension E or S in the catalogue. Too small a gap lets shaft thermal growth squeeze the insert, while too large reduces jaw engagement and loads the insert unevenly. Measure it with feeler gauges after alignment.

Jaw, tyre and disc couplings need no lubrication. Gear and chain couplings do need a dedicated grease and periodic inspection, often every few months depending on load. Grease starvation in a gear coupling wears the teeth quickly and causes vibration.

Flexible couplings only accept small misalignment within catalogue limits and do not replace proper alignment. Running near the limit heats the insert, shortens its life and passes extra load into the bearings. Align so that only a small fraction of the coupling's capacity is used.

Common causes are misalignment, a worn insert leaving backlash, a coupling half loose on its shaft, or imbalance. Vibration at twice running speed usually indicates misalignment, while vibration at running speed usually points to imbalance. Check the runout of the coupling halves and the key condition before replacing the coupling.