K21 brake parts

Ken Reynolds wrote:
"Stuart Kinnear" wrote in message
...
Does anyone know of an automotive source for the complete brake caliper
Stuart
Tost Catalog says K21 uses Cleveland wheel . It is reasonable to assume
its also a cleveland brake.
Aircraft Spruce among others sell Cleveland stuff
Ken Reynolds

I agree with the other postings, do not try to mix automotive parts
with the K21 Cleveland wheel brakes. The disc is expensive and should
be replaced when it gets too thin (the limit is in the manual). If the
pads are worn and the disc is out of limits the pistons can extend too
far out of the calliper and pop the seals, leaking fluid, spilling onto
the tyre, which does in no good at all. The brake system then requires
a complete rebuild.

We have three K21s, two do about 800hrs per year and the other one
about 200hrs per year. The discs are replaced about twice per year and
the pads about three times per year. Having a spare wheel, calliper and
brake pads speeds up the process. Be aware that you can get long life
pads, but they must not be used on stainless steel discs. I keep an old
worn disc that has been worn down to the minimum limit that makes
checking wear easy. The pads are easy to replace, each pad is held by
three rivets and can either be drilled out or punched out. The new set
of pads are fixed with new rivets, supplied with the pads and fitted
either will a screw clamp press or a hammer and punch set. Beware the
pads are rather brittle.

Removing the calliper is easy but the bolts must be wire locked with
stainless steel locking wire. The whole system must be bleed. It is
best to use a large syringe.

Note that the linkage needs to be adjusted so that full air-brake can
be achieved and that sufficient movement is applied to the master
cylinder.

As the pads can be used of military aircraft, they required an "end
user certificate waiver" and are now subject to International Trade in
Armament Regulations (ITAR). No Joke.

Removing the wheel requires thin walled sockets (I think 17mm from
memory), I have a ground down set in my toolbox.

Replacing the wheel bearings is also rather problematic, they are cone
roller bearings. The seats have to driven or pressed out. When the new
seats are installed the gaps have to be adjusted will shims to
eliminate almost all of the sideways float. The wheel is in two halves
joined by six socket headed high tensile steel bolts, these must be
torqued to the required setting (in the manual for the wheel), it is
best to replace the nylock nuts every time the wheel is taken apart. To
avoid nipping the tube, partiarly inflate the tube during assembly. The
standard tube valve protrudes too far and can fowl the brake, so short
lengths of tube (fuel pipe) are used to provide clearance. The same
trick is used on the nose wheel and the tail wheel. The wheels can also
corrode so I give them a light coating of a silicon or PTFE spray (GT
85 as used on bike chains is very good). We replace the main wheel
bearings and tyres every three years after about 2,000 - 2,500hrs,
12,000 to 15,000 launches and landings.

Our club does about 10,000 launches per year (90% by winch) and we land
on grass. The nose wheel pressure has been reduced and the tail wheel
pressure increased in order to minimise damage to the glider.

If the nose wheel bearings stick the wheel rotates on the fixed axel
and as this is a metal on metal surface wears quickly and makes a lot
of noise on the winch launch (the high speed winds up the wheel and
requires the glider to be flown very slowly before it stops). The fix
is to use some sot of bearing locking solution when the axel is fitted.
The tip is to fit the axle, push it out an inch apply the solution,
push it in and pull it out until enough gets onto the portion of the
shaft in the bearing and tighten it up. Leaving the glider with the
wheel off the ground to cure helps.

We tried to minimise the damage to tail wheels by filling them with
foam; it just transferred the damage to the rest of the glider! AS was
about to test this fix when our technical officer (Joe Fisher) advised
them that it did not work. (Joe designed the rear seat hand rudder
modification for the K21, our gliders can be operated by hand rudder
from either seat).

The only solution to damaging the tail wheels is to increase the
pressure in the tail wheel and to land properly. Ensuring that the
glider is balanced on the main wheel, when turning the glider helps to
reduce damage to the sidewall of the tyre.

I will dig out the standard pressures and what we use now.


Chris

p. s. I was the Director for gliders, safety officer and vice-chairman,
at our club and currently I am our full time weekly course instructor;
so K21 HPV is my office. In the UK our agent is Peter Wells at
Zuluglasstek, who is very helpful if anyone encounters unusual
problems.


NB The DG505 etc also uses automotive brake fluid DOT3/DOT4. Never use
the wrong fluid as it eats the seals.