Assembling The Main Mechanics - GRAUPNER heim UNI-Mechanik 2000 Mode D'emploi

UNI-Mechanics 2000

1. Assembling the main mechanics

Most of the components of the UNI-Mechanics 2000 system are moulded in glass fibre
reinforced nylon, a composite which offers important advantages over other materials such as
aluminium, which are used to build model helicopters. These advantages are low weight
combined with high mass constancy, freedom from fatigue effects, low noise, and the ability to
absorb motor vibration. Careful design of these mechanical systems gives them the robustness
and rigidity required; when a helicopter suffers a „hard landing" it is most helpful to the pilot if
the parts either survive undamaged (in which case they can be used again without restriction),
or alternatively simply break, so that there is no doubt that they have to be replaced. This
construction eliminates the problem of bending or distortion of the chassis, which certainly
affects metal mechanics; such damage may even go unnoticed by the owner, but it can easily
cause other components to fail prematurely, and has a negative effect on the entire system's
efficiency and safety.
Compared with its many advantages, the only drawbacks to nylon construction are the greater
(and more cost-intensive) complexity in manufacture, together with the requirement for the
owner to assemble the parts carefully and conscientiously, and to adjust and trim parts slightly
where necessary. The system then repays you for your care with a long-lasting model and low
rates of wear.
Shafts, bearings, fits
Virtually all the rotating parts of the mechanical system are ballraced. Where ballraces are used
it is very important that the shaft is a tight fit in the inner ring of the bearing, otherwise there is a
danger that it will rotate within the ring. This causes the inner ring to heat up (a blue or yellow
discoloration betrays this), and the bearing is damaged and has to be replaced. In an extreme
case the bearing may become so hot that it melts the nylon bearing seat, and the shaft then
loses its correct position relative to other components. If this should occur, it is tempting to claim
that the bearing seat material is at fault, but the actual problem is the incorrect shaft/bearing fit.
If a bearing fit is too loose, another possible consequence is that the inner ring causes wear on
the shaft, and the resultant local reduction in shaft diameter causes a change in meshing
clearance in any gears mounted on the shaft, i.e. the gears wear rapidly and eventually fail
altogether.
In order to avoid the problems described above, the fits between shafts and ballraces are
maintained on the close side of normal in the Graupner/Heim system. If the manufacturing
tolerances in the bearing and shaft should be unfavourable, you may occasionally find that the
fit is too close, i.e. the bearing cannot be pushed onto the shaft. In this case the shaft must be
reduced in diameter by rubbing with fine abrasive paper (600 - 1200 grit) until the bearing can
be pushed into position using no more than moderate force.
At the other extreme, if the combined manufacturing tolerances produce too loose a fit - as may
also occur occasionally - the solution is to glue the bearing to the shaft using LOCTITE bearing
retainer fluid 603, which fixes the parts together reliably. When you use this material, please
note that the cure time of the fluid varies with the closeness of the fit: the closer the fit, the faster
the cure. Under certain circumstances you may only have a few seconds to locate the bearing
correctly on the shaft before it is fixed immovably.
If a shaft is supported in multiple bearings, it is important to prevent axial stress due to incorrect
positioning relative to each other. One way to avoid this is to position both bearings on the shaft
really accurately; an alternative is to use a combination of fixed and sliding fits: one bearing is a
press-fit on the shaft (or is glued in place), and the other bearing is a sliding fit, i.e. it can be
shifted axially on the shaft using moderate force. In the latter case the second bearing takes up
the optimum position automatically once installed.
The problem of shafts wearing inside bearings is affected by two factors: the smaller the shaft
diameter and the higher the rotational speed, the greater the danger. The problem of stress
between bearings is affected by the difference in internal and external diameter of the bearings:
the smaller the difference, the greater the danger. Bearing fits are important, and if you wish to
produce a helicopter which is as safe and reliable as possible, all these factors have to be taken
into account in every individual case. For this reason the building instructions state in every
case when the bearings must be secured using thread-lock fluid or bearing retainer fluid.
7