Until the advent of frictional drives (see below), kilns only ever had one turning gear and this supplies all the torque to turn the kiln, so in the case of a long kiln, it is usually positioned somewhere near the middle (strictly speaking, the centre of mass) to minimise the amount of torsional distortion produced in the shell. Preferably a relatively cool section of the kiln is chosen. The gear is placed near to a tyre so that it is accurately aligned with the kiln axis, with minimal wobble. It is normal for the nearby tyre to be fixed in position with thrust rollers, so that as the kiln expands on warming up, the turning gear position remains fairly constant, while the nose and tail of the kiln expand outward. The pier of the nearby tyre is usually extended to include the pinion mounting bed, the gearbox and the motor, although on early kilns it was common to mount the motor on the kiln house floor, and connect it to the gearbox with a flat belt. In the case of shorter dry process kilns with preheaters, it has been normal practice to locate the turning gear next to the rear tyre, at the coolest part of the kiln.
Early turning gears were attached directly to the shell. Differential expansion in such circumstances causes the gear to break and the kiln shell to "neck", and this practice was soon abandoned in favour of some sort of flexible mounting. Various mountings have been used, but by far the most common is the tangential mounting, which emerged in the first decade of the twentieth century. Flexible plates are riveted (and later welded) to the kiln shell tangentially, the other end being fixed to the gear ring through a flexible coupling. This allows expansion of the shell to take place unfettered. It also results in minimal heat transfer to the gear ring, so that the latter remains cool enough for conventional lubricants to be used. Tangent plates must operate in tension, and so they differ from longitudinal mountings in that the kiln may not be run in reverse.