fit is less reliable, because if it loosens, the entire fit is free with but little axial movement. Some lubricant, such as white lead and lard oil mixed to the consistency of paint, should be applied to the pin and bore before assembling, to reduce the tendency toward abrasion. Assembling There are two basic methods for assembling an oversize shaft into an undersized hole, sometimes used in combination: force and thermal expansion or contraction. Force There are at least three different terms used to describe an interference fit created via force: press fit, friction fit, and hydraulic dilation. Press fit is achieved with presses that can press the parts together with very large amounts of force. The presses are generally hydraulic, although small hand-operated presses (such as arbor presses) may operate by means of the mechanical advantage supplied by a jackscrew or by a gear reduction driving a rack and pinion. The amount of force applied in hydraulic presses may be anything from a few pounds for the tiniest parts to hundreds of tons for the largest parts. The edges of shafts and holes are chamfered (beveled). The chamfer forms a guide for the pressing movement, helping to distribute the force evenly around the circumference of the hole, to allow the compression to occur gradually instead of all at once, thus helping the pressing operation to be smoother, to be more easily controlled, and to require less power (less force at any one instant of time), and to assist in aligning the shaft parallel with the hole it is being pressed into. In the case of train wheelsets the wheels are pressed onto the axles by force. Thermal expansion or contraction Most materials expand when heated and shrink when cooled. Enveloping parts are heated (e.g., with torches or gas ovens) and assembled into position while hot, then allowed to cool and contract back to their former size, except for the compression that results from each interfering with the other. This is also referred to as shrink-fitting. Railroad axles, wheels, and tires are typically assembled in this way. Alternatively, the enveloped part may be cooled before assembly such that it slides easily into its mating part. Upon warming, it expands and interferes. Cooling is often preferable as it is less likely than heating to change material properties, e.g., assembling a hardened gear onto a shaft, where the risk exists of heating