Winch Launch Stresses on Vintage Gliders

It is standard practice to use two weak links in parallel. One has slotted
holes, so there is no strain on it unless the primary weak link fails. If
there is a true overload, both weak links fail. However, if the primary
weak link fails due to fatigue, but the actual forces are below the weak
link limits, the secondary weak link does not fail, preventing an
unnecessary launch failure.

However, if you screw up and use two identical weak links (either both with
regular holes, or both with slotted holes) you have the problem you alluded
to, which is that you effectively have a single weak link with double the
intended strength.

Mike Schumann

Agreed, however we have found that gust loads that break one, generally
break two, and we've taken to a policy of using only one TOST dog bone
at a time. Recently at the beginning of the day we found someone had
assembled a TOST assembly with two dog bones. The chief instructor
took it apart and found two round holed links. We think a single dog
bone of the proper rating is sufficient.

Frank Whiteley

Is it possible to see that one is slotted and one round holed when the
cable is ready to be hooked up to the glider? Surely the pilot must
be given the opportunity to inspect the links before accepting the
cable. It's a while since I winched and we used rope weak links then.

Andy

Andy, that's a good question. Actually, as far as I know, there isn't a way
to visually inspect then once they are assembled into the protective case.
One wonders why.

It would be easy to dip one end of the slotted links in, say, tan paint.
Seeing a link assembly with two white dogbones but with one showing a tan
end would indicate that the link had been assembled correctly.

Bill Daniels


"Andy" wrote in message
oups.com...
Is it possible to see that one is slotted and one round holed when the
cable is ready to be hooked up to the glider? Surely the pilot must
be given the opportunity to inspect the links before accepting the
cable. It's a while since I winched and we used rope weak links then.

Andy

We use a system where the weak link is held in an H channel with one end
hole drilled and the other slotted so that the end bolt is supported but
can pull out when the link breaks.

This gives a good protection to the link. We found that the metal
shrouds bent and also you couldn't see the link colour. With the H
channel you can see the whole link.

Robin

In message , Bill Daniels
writes
Andy, that's a good question. Actually, as far as I know, there isn't a way
to visually inspect then once they are assembled into the protective case.
One wonders why.

It would be easy to dip one end of the slotted links in, say, tan paint.
Seeing a link assembly with two white dogbones but with one showing a tan
end would indicate that the link had been assembled correctly.

Bill Daniels


"Andy" wrote in message
roups.com...
Is it possible to see that one is slotted and one round holed when the
cable is ready to be hooked up to the glider? Surely the pilot must
be given the opportunity to inspect the links before accepting the
cable. It's a while since I winched and we used rope weak links then.

Andy




--
Robin Birch

On Sun, 29 Jan 2006 15:35:45 UTC, "Bill Daniels"
bildan@comcast-dot-net wrote:

: It would be easy to dip one end of the slotted links in, say, tan paint.
: Seeing a link assembly with two white dogbones but with one showing a tan
: end would indicate that the link had been assembled correctly.

I'm pretty sure that I have seen genuine Tost weak links in which the
slotted ones had a paint band round the middle. But that may have been
added by the club in question. It's a sensible thing to do.

Ian

Andy wrote:
Is it possible to see that one is slotted and one round holed when the
cable is ready to be hooked up to the glider?

Easy: Pull at both ends. If everything is correct, then one link is now
fixed while the other can be moved freely.

And that's the idea of the arrangement: Over time, the weak link will
stretch, even when used within its load limit, and therefore weaken. In
other words, after some time, the link will be weaker than advertised.
The second link however will keep his strengh, as it never takes load.
So when the first link breaks prematurely, the second will still hold
the advertized load.

When you pull at both ends and one link can't be moved freely under the
load, then the first link has stretched and must be changed.

Stefan