A drift pin is a device used to retain a pulley or gear onto a shaft. The drift pin is a small and often hollow pin made of tool grade steel. It is placed in a hole drilled through the pulley or gear's flange and driven into a hole drilled through the drive shaft with a hammer.
Often, a drift punch is used to permanently locate the drift pin into position. The drift pin is typically hollow and split down one side; this allows the pin to compress when driven into the hole in the shaft that is a fraction smaller than the diameter of the drift pin.
The drift pin is lodged into the shaft and holds the gear or pulley into position by nothing more than an interference fit. There is nothing beyond the tight fit of the pin into the shaft to maintain the coupling. In many instances, the pin is designed to fail if an interference between the gear or pulley would happen to occur. By breaking the drift pin, the machine is protected against further damage as the mechanical device is allowed to turn uninterrupted. By removing the obstruction and replacing the pin, the machine is allowed to assume normal operations.
The drift pin method of securing an object to a shaft is much more reliable than attempting to secure a gear or pulley with a bolt. Tightening a bolt against a drive shaft will ultimately result in a slipping situation. The bolt's square face is not a proper match to the round shape of a shaft and machining a flat surface in the shaft removes some of the shaft's strength.
Used extensively on farm machinery as well as manufacturing machinery and conveyor systems, the drift pin remains an effective and easily maintained method of securing equipment and machine components together. In the event that a gear swap is called for, simply driving the pin out of position, removing the gear, placing a new gear onto the shaft and installing a new pin is all that is required. The equipment is repaired with a minimal amount of down time.
From huge harvesting machines found on the farm to the smallest fishing reel, drift pins are incorporated to secure integral parts when they cannot be allowed to move. By never requiring adjustment or lubrication, the drift pins provide a valuable service with a minimal amount of effort.