How to shorten a Tach drive cable?

"Morgans" wrote in message
...

It has been pointed out that there are specialty shops that call
themselves a speedo shop, and I was surprised that anyone could make a
business on such a small subgroup of auto repairing. I stand corrected,
but I have never heard of such a thing where I have lived.

Speedo repair is/was a huge business. All taxicabs (at least they used to)
had to have their meters tied in with the speedo/odo and calibrated at
regular intervals. Many commercial vehicles have "Speed-o-graphs" or
"Tach-o-graphs" which record speed or rpm for monitoring by employers or
owners. Then there is the occasional aircraft repair job. We had several
such shops in Seattle. One of their services (I remember well) was to
provide error reports for drivers who claimed in court that, "Honest
officer - my speedometer only showed 50! I couldn't have been doing 73. . ."

Rich S.

Another question, how important is the twist (lay) of the cable?
There are left lays and right lays depending which direction the cable
spins.
I talked to one shop and they didn't think that it really made a
difference since they solder/braze the end of the cable. They only had
left lays in stock.

Any opinions?

Dave

Dave wrote:
Another question, how important is the twist (lay) of the cable?
There are left lays and right lays depending which direction the cable
spins.
I talked to one shop and they didn't think that it really made a
difference since they solder/braze the end of the cable. They only had
left lays in stock.

Any opinions?

Dave

From the "Flexible Shaft Handbook" which just happened to be sitting
on my desk today:

"It can be readily understood that a shaft will transmit its maximum
capacity when the rotation tends to tighten up the outer winding.
Consequently .... a left lay shaft should be used for driving in a
clockwise direction. For the same reason, a right lay shaft should be
used for counter-clockwise driving, always looking at the shaft from the
supply end

While flexible shafts deliver their maximum capacity when operating in
one direction only, they may be used with entirely satisfactory results
for driving in both directions.... either a left-lay or right-lay shaft
may be used, but it must be of a larger size...

A shaft working under somewhat less than full load capacity or under
conditions of small loads, will operate equally well in either direction."


The handbook has tables of shaft diameter and radii to compute maximum
torque, and a de-rating factor to apply when running against the lay. A
1/8 inch shaft with a 4" radius bend de-rated for reverse lay is still
good for better than .25 inch pounds of torque, with a twenty inch
radius, it is good for 1.6 inch pounds and with no curve, 2 inch pounds.
The book is very dated, but I expect that it is probably conservative.

From my personal perspective, I have found that keeping it well lubed
and maintaining large radii is pretty important when running against the
lay, as friction and wear is increased with low lube and tight radii,
and running against the lay can lead to a worn area sprouting wire hairs
that jam up good in the sleeve.

Charles