The advantages of roller bearings compared with plain or gliding bearings.
The Jefa rudder bearing range is almost completely based on roller bearings (except for some heel bearings for skeg rudders). We strongly believe that roller bearings are the only bearings that should be used in rudder systems. The advantages of roller bearings compared with plain or gliding bearings are as follows:
Extremely low friction: Even in the worst sailing
conditions, roller bearings will guarantee a smooth rotation of the
rudder shaft. Roller bearings provide an exceptionally low friction
co-efficient, typically less than 1/10 of that of a plain bronze or plastic bush
Nearly no backlash: There is virtually no backlash between
the rudder stock and the Jefa roller bearing. In a conventional bearing, it's necessary to provide a running clearance to avoid excessive friction.
This means that the inside diameter of the bush always has to be a couple of
tenths of a millimetre bigger than the outside diameter of the shaft to
guarantee rotation. On top of that there is a big difference in plastics
suitable for gliding bearings and for roller bearings. The plastics suitable
for gliding bearings mostly absorb much more water than those suitable for
rollers. This growth has to be compensated by extra initial backlash in the
gliding bearing. If the backlash is too small, the bearing will seize in
time, allowing no rotation of the rudder stock any more. As it is
unpredictable how much the bearing will grow (this depends on temperature,
water type, the use of lubricants, etc.) , one tends to drills the hole in the bush
to an oversize, creating even more backlash.
The material used for the rollers in Jefa bearings is called Polyacetaal or Polyoxymethyleen (POM), mostly identified by it's trade name Delrin. Practical test and inspections on boats that have been continuously in the water for more than 5 years have shown a relative growth of the rollers of less than 0.05 %. Due to the construction and materials used in the Jefa bearings, the allowed backlash between the shaft and the bearing is between 0.03 and 0.05 mm. This extremely small backlash (in practice not noticeable) will stay constant over the lifetime of the sailing yacht as due to the roller arrangement no material on the shaft and bearing is lost in time.
A longer lifetime of the rudder shaft: One off the big advantages
of the extremely low friction co-efficient of the Jefa roller bearings is
the fact that there is no surface rubbing present in the bearing. The
rollers will always roll smoothly over the shaft surface, leaving it untouched,
and guarantee the high tolerance between the shaft and bearing. Inside
gliding bearings the contrary happens: Heavy constantly surface rubbing will
take place between the shaft surface and bearing surface. This will not only
mean that the bearing will loose material in time, but also the shaft will
run in and loose material. With stainless steel shafts, this material loss
in time will be a couple of hundreds of mm, with aluminium shafts the
material loss is more dramatic. This is due to the difference in protection
against corrosion of the the different metals. Stainless steel is non
corrosive by its mixture of alloy elements. Aluminium is protected against
corrosion by a thin layer of aluminium and silicone oxide on the shaft
surface. Every time the shaft is rotated under load, a bit of this oxide
layer is scrubbed off. The material will react instantaneously by forming a
new layer of oxide, sacrificing some shaft material. In time this process
could diminish the shaft diameter will a couple of tenth of a millimetre.
Replacing the bush of the gliding bearing will not solve the backlash any
more as the shaft is too heavily damaged. The only solution is to put a
sleeve over the shaft with an oversized bearing. (For details see our conversion
Easy to replace: The replacement of the bush in a gliding bearing should be done every couple of years to keep the system at a minimal amount of backlash. This is normally a heavy job as the bush is pressed into the tube, and after swelling it will be very difficult to get it out. Roller bearings normally don't have to be replaced. The only instance thinkable would be if the rollers can't rotate due to dirt falling into the bearing. Replacing the bearing is very easy and described in our maintenance section.
The construction of Jefa rudder bearings:
All Jefa bearings are produced on the high quality CNC (Computer Numerical Controlled) machines, guarantying a constant high quality. Please take a look on the Jefa info section for further details. All aluminium parts are made of seawater resistant aluminium AlMgSi1 (see our materials section for further details on this material) and are black anodised after machining. The balls in the self aligning bearing, the rings underneath the bearing and the roller tracks are made of white Delrin. The rollers are made of high precision black delrin bar.
Why do Jefa bearings run directly on the shaft without an inner housing?
Some other producers of rudder bearings use an extra housing between the rollers and the rudder shaft. Twenty years ago, Jefa Marine took the decision not to use that system as we can only see downsides with this system. The big problem with that extra inner housing is the control of the backlash between that housing and the rudder shaft. Any backlash in a rudder bearing is multiplied by the long distances between the bearing and the connecting steering lever. The mechanical reduction of the steering system itself also heavily multiplies the backlash. A small backlash of for example 0.1 mm in the rudder bearing could result in a backlash of a couple of centimetres on the steering rim. Independently of the solution chosen, the rudder shaft tolerance has to be very precise. Lets take an example of a 90 mm rudder bearing arrangement. The bearing inside diameter will have to be made to 90.00 millimetres. The rudder shaft will have to be 89.95 to 89.97 millimetres. Only by producing both items on this extremely high tolerance one would get a satisfactory solution. Now we get to the difference in bearings with an inner housing and the Jefa bearings: The mounting on the shaft. A bearing with an (aluminium) inner housing with the above tolerances will in practice be nearly impossible to push over the shaft. The dimensions as given in the above example are theoretical as every dimension has a tolerance. Sometimes both tolerances will be at the wrong side. (In our example the bearing would have an inside diameter of 89.98 and the shaft would have an outside diameter of 89.97). A bearing with a solid aluminium inner housing would not be mountable any more. The Jefa bearing however would be a bit stiff to mount, but it will be reasonably easy to do. This is due to the fact that we advise in our installation instructions to rotate the bearing on the shaft while pushing it to the correct place. Due to this rotation, the rollers will twist a bit in vertical direction and allow the bearing to be "screwed" on. Due to this phenomena, Jefa can produce the bearings with a much tighter tolerance compared to inner housing bearings. Often the producers of inner housing bearings put multiple O-rings inside this inner housing to camouflage the backlash present, but as soon as any loading will be put on the bearing, the backlash will be present with all associated irritations.
The difference between self aligning and non self aligning bearings:
|Under load, every rudder stock will bend in proportion
to the forces applied. The amount of deflection will vary according to
the size and the material of the rudder stock and the position of the
bearings. When this deflection is high, it causes very high point
loading on a non self aligning bearing, which results in excessive
friction, and in the case of plain bearings, self locking. The friction
is not always correctly identified, but put down to either hydrodynamic
forces on the rudder blade or inefficient steering. Where the plain
bearings are replaced with self aligning roller bearings, the steering
characteristics are transformed, and what may have been considered as
heavy steering suddenly becomes so light that the steering can actually
be made more direct by reducing the wheel turns.
This doesn't necessarily mean that that self aligning roller bearings are the only good solution. On cruising yachts, with relatively big rudder stocks and the bearings not too far apart, normal roller bearings could also provide a satisfactory solution.
The ease of installation is also an important factor in the choice between normal roller bearings and self aligning roller bearings. The self aligning type may be more expensive to purchase, but one saves precious and expensive installation time as the self aligning type can be installed in the hull at a very early stage without the rudder stock present for alignment. The standard roller bearings can only be installed at a stage where it is possible to put the rudder in the hull as the bearings will have to be carefully aligned to the shaft. For details see our installation in GRP or wood epoxy hulls section.