The rapson slide acting on either a fork tiller or the more common round arm. The tiller drives the rudder stock by means of a key or keys. The crosshead is free to slide along the circular arm of the tiller so that the straight line effort of the rams is applied to the angular moving tiller. Each set of two cylinders in line are connected by a strong steel girder usually called a "Joist" which stiffens the system and forms a "guide bar" for the crosshead guide slippers to slide along. The joist is often designed to incorporate the steering engine stops.
An important consideration in all steering gears is the "wear down" of the rudder carrying bearing, this bearing takes all the weight of the rudder. Therefore there must be adequate clearance between the bottom of the tiller and the crosshead bearing, so as the rudder bearing wears down in service the tiller and crosshead bearing do not touch, clearance when new can be 22 mm at bottom and 12 mm at top; the top clearance is a precaution to stop the tiller bumping up the steering rams in the unlikely event of the rudder lifting in heavy weather. Should the bottom of the tiller and the crosshead bearing touch, then the weight of the rudder will be transferred from the rudder bearing to the steering rams with disastrous results such as leaking of working fluid from the cylinders and shearing of the rams.
In the case of forked tiller design, the thrust from the rams is transmitted to the tiller through swivel blocks. One advantage of this arrangement is that the overall length of pairs of rams is reduced compared to the round arm tiller design and this can be an important consideration in some cases. A disadvantage is that where as any slight misalignment in the case of the round arm tiller is not vitally important, it could lead to uneven loading of the swivel blocks in the forked tiller design and it is essential that the line of the rams be exactly at right angles to the rudder stock centre line if this is to be avoided.
With the Rapson Slide the torque reaction from the rudder is taken on the tiller by a force which is balanced by an equal and opposite force having two components one of which is produced by the ram and acts in the line of the ram, whilst the other is at right angles to the line of the ram and is produced by the guide reaction.
Where guides are not fitted as is sometimes the case with smaller steering gears then the guide reaction force must be carried by bearings or the glands of the cylinders.
a = actuator area
p = Working fluid pressure
n = Number of effective rams ( 1 for 2 ram, 2 for 4 ram)
q = rudder angler = tiller radius at amidships
r' = tiller radius at qo of tiller helms = guide reaction force
f = force on ram with tiller amidships ( = p x a)
f' = effective force acting at 90o to tiller
r' = r / cos.q also f' = f / cos.q = p x a / cos.q
t = torque available = f' x r' x n= ((p x a) / cosq). (r / cos.q) . nt = (p x a x n x r) . (1 / cos.2q)
Showing that the rapson slide effect which gives increase of available torque with increases of rudder angle
The torque demanded from the steering gear increases and is at a maximum at maximum rudder angle when the mechanical advantage of the Rapson Slide gear is at a maximum. Ram type gears are also well adapted to take advantage of the high pressures which are currently available, since ram diameters and casing are relatively small and leakage paths are small or non-existent.