GRAUPNER Skalar, Manual

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elektro
Skalar
7
1. Assembling the main mechanics
The mechanics consist largely of glass fibre reinforced nylon, a material which offers consider-
able advantages for model helicopter construction compared with, say, aluminium. These ad-
vantages include high mass constancy combined with low weight, freedom from fatigue effects,
low noise when operating, and the ability to absorb motor vibration. The design of this type of
mechanical system endows it with great robustness and rigidity; in a hard landing it is preferable
that the parts should either survive undamaged (and therefore be unconditionally re-usable) or
simply break, in which case they have to be replaced. In contrast, if the chassis should bend or
warp, such damage might not even be noticed, but it would certainly have a serious adverse ef-
fect on the other components, which might not work at all, might fail prematurely, or could even
jeopardise the safety of the whole system. These are problems inherent to metal mechanical
systems, but they do not occur with our type of construction.
The many advantages of reinforced nylon construction are balanced by just a few drawbacks:
the parts are more complex (and therefore more expensive) to manufacture, and the builder is
required to be more careful and conscientious when assembling and setting up the components;
slight trimming of the parts themselves may also be necessary occasionally. The reward for a
considered, patient approach is a model which is very durable, wears very slowly, and therefore
has an extended useful life.
Shafts, bearings and fits
Virtually all the rotating parts of the mechanics are ballraced. When ballraces are used it is very
important that the shaft is a tight fit in the inner ring of the race, so that it cannot revolve within
the ring. Otherwise the inner ring heats up (discolouring it blue or yellow), damaging the bearing
and rendering it unusable. In the worst case the bearing may become so hot that it melts the
nylon seating, and the shaft then loses its position relative to other components. Please note that
this is not a fault in the bearing seat material; it is simply a result of incorrect bearing fits.
A further possible result of a loose bearing fit is that the shaft slips within the inner ring, and its
diameter is worn down in the bearing area. In this case the carefully set meshing clearance of
any gears mounted on the shaft is lost, i.e. the gears no longer mesh correctly, leading to in-
creased rates of wear and eventual failure.
In the Graupner/Heim system the fits between shafts and ballraces are maintained on the tight
side, in order to avoid the problems described above. However, manufacturing tolerances are
inevitable, and occasionally the result is too tight a fit, i.e. the bearing cannot be pushed onto the
shaft. In this case the shaft must be rubbed down using fine abrasive paper (600 - 1200 grit) un-
til the bearing can be pushed onto it using no more than moderate force.
If the fit should be loose - which can also arise with the accumulation of manufacturing toler-
ances in both parts - the bearing can simply be glued to the shaft using LOCTITE bearing re-
tainer fluid 603, which guarantees a firm seating. Please note that the cure time for bearing re-
tainer fluid varies with the fit: the tighter the joint, the quicker it cures. Under certain circum-
stances you may only have a few seconds to locate the bearing correctly on the shaft before it is
permanently and immovably fixed.
If a shaft is supported in multiple ballraces, it is important to prevent the bearings being under
tension in the axial direction. This can be achieved in either of two ways: either by locating both
bearings on the shaft very accurately, or by using a combination of fixed and sliding fits: one ball-
race is a press-fit on the shaft, or glued in place, making it immovable, whereas the other bear-
ing is a sliding fit, i.e. it can be moved along the shaft axially using moderate force. It will then
take up its optimum position automatically when installed.
In general terms you can assume that the smaller the shaft diameter and the higher the rota-
tional speed, the higher the danger of wear in bearings.
The smaller the difference between inside diameter and outside diameter of the ballraces, the
higher the danger of tension in the bearings against each other.
If your model is to be as safe and reliable as it possibly can be, then all this needs to be taken
into account whenever you are fitting a ballrace to the model. For this reason the building in-
structions always state when thread-lock fluid or bearing retainer fluid must be used for the vari-
ous joints and connections.