In common axles, the wheel is prevented from coming off by a pin, called the linch pin, passing through the end of the axletree arm, the name of the part that the wheel turns upon; but as many serious accidents have happened through the linch pin failing and the wheel coming off, an improved method of securing the latter is now practised, by means of a box called the axletree box, which is contrived to answer the double purpose of keeping on the wheel, and to hold oil, grease, or some lubricating substance for lessening the friction.