Selected models of jaw coupling hubs feature a cutout slot in the mating part (hub) of the couplings, known as keyways. These slots or keyways fit in the keys or a separate mechanical attachment on the shafts or other mated tools. The use of "Keys" leads to improved torque & prevents slipping. It also results in reduced losses due to vibrations & backlash (degree of free movement between two rotating, mating parts).
However, some applications do not require keys, as keyless jaw coupling hubs use only set screw holding power to transfer torque. Keyless hubs are ideal for applications that cause no damages or injuries even if the shafts slip. Keyed jaw coupling hubs concentrate the stresses around the key or key slot, causing maintenance issues and efficiency losses. Keyless hubs distribute the load & hence require less maintenance.