The firing (and compression) forces are transmitted through the crosshead into the connecting rod so that, apart from TDC and BDC, a turning moment is developed at the crankshaft level. There will be a transverse reaction to these forces at the crosshead level, and this is taken by the guide shoes (slippers) onto the guides. A simple triangle of forces shows this reaction for one point alone in the cycle, for as the cylinder pressure changes so too does the loading on the piston.

In other words, the load on the crosshead, and hence the guides, will vary throughout the stroke with variations in crank angle and cylinder pressure. Not only does the magnitude of the loading vary, but its direction of application alters too. During the expansion stroke the forces will be acting on one set of guides (sometimes referred to as the ahead guides) then during the completion of the revolution the thrust is transferred to the other set of guides (the astern guides). From this it should be appreciated that both sets of guides (ahead and astern) are used every revolution, and not only the ahead and the astern guides when the engine is running astern. Both set of guides are used whether running ahead or astern. There will be a greater loading on them during the relevant power stroke than that experienced during the compression stroke.
Wear, under normal circumstances; occurs along the top third of the guide ways and it will generally be slight compared to the wear on the slipper itself. This, being white metal faced and provided with grooved oil reservoirs, is supplied with oil bled from the crosshead, as mentioned above.
Clearances should be checked using feeler gauges, at the top, centre and bottom of the stroke, with a total clearance (ahead and astern) being taken, as the guide shoe may be resting on either of the two surfaces.
Adjustment of the guides to compensate for wear should be undertaken with great care and any adjustment made should be in order to recover the smallest wear measured and not the largest (otherwise guide clearance may be lost on the least worn areas). 
Alignment of the guides should be such that they control/constrain the crosshead, and therefore the extremity of the piston rod; to a path parallel to the movement of the piston. This runs according to the alignment of the cylinder liner so the guides should be parallel to the cylinder liner. Any deviation from this will, apart from producing a knocking sound, give accelerated wear to the cylinder liner, piston rod and possibly the guide shoe and ways.
Adjustment of the guide ways and accurate measurement of the alignment is not usually undertaken by the ship’s staff. However an impression of the alignment can be obtained, should some fault be suspected and all other possibilities eliminated, by adopting the following procedure. With the cylinder cover removed and the piston suspended (crane) at the lowest part of the liner, centralise it within the unworn part of the liner by driving in wooden wedges (circumferentially and from both top and bottom of piston). This aligns the piston with the cylinder liner and in so doing makes the rod (freed from crosshead constraints) parallel to the liner. Measurements can then be taken from rod to guides to check for any deviation. More accurate methods, using piano wire centralised to cylinder bore, or laser beams, are available, but the above procedure will indicate any major misalignment.
When adjusting these to compensate for wear, the effects of any adjustment on the alignment of the piston rod should be considered. The crosshead may be placed transversely thereby pulling the piston rod end to one side of the engine. The two faced guides are most likely to give problems in this way unless correctly adjusted.
The guides, absorbing the resultant transverse, thrust from the crosshead, apply a corresponding turning moment to the engine in the transverse plate about the base. The forces so applied tend to ‘rock’ the engine (transversely) about its foundation. This increases the compressive loading on the chocks of the relevant unit on one side of the engine, whilst simultaneously increasing the tensile stress in the holding down bolts at the other side of the engine. The compression stroke reverses these forces, but not by the same magnitude. Compared to the firing forces and vertical forces in general these transverse, rocking moments are small. Nevertheless, their effects at holding down bolt level should not be disregarded. The combination of otherwise acceptable individual loadings may cause failure. For example, consider the effects of a higher than normal firing pressure and slack or broken chocks on the rocking forces mentioned above.

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