<VV> pulleys in tunnel

[Roger Gault]

Hi, Dorai,
The clutch cable that runs over the larger pully is a stranded steel cable.
These cables have a minimum allowable bend radius.  The smaller the bend
radius of the cable around the pulley, the higher the stresses in the cable
as it bends around the pulley.  Every time you move the clutch pedal, the
bend in the cable moves up and down the cable, so the stress at any one
point cycles between some maximum and zero.  This results in metal fatigue
in the cable strands.  The higher the stress, the quicker the cable strands
will fatigue and break.  If you go to the smaller pulley, you'll bend the
cable too tightly and cause it to fail MUCH earlier than it would if using
the bigger pulley.

The emergency brake cables are much smaller in diameter, so they have a much
smaller minimum bend radius.  That, and the fact that they don't move around
the pulley every time you shift gears allows the pulleys to be smaller.

Also, I had both of these pulleys in my hand weekend before last.  I wasn't
looking for this kind of information at that moment, but I suspect that the
groove in the brake cable pulley is too narrow for the clutch cable, and the
cable won't run in the bottom of the groove.  That will split the pulley
quickly.

Get the bigger pulley.

Roger Gault
BS Mechanical Engineering, Texas A&M University, 1969

PS:  A quick story:  When I was growing up, my dad was an aircraft (Piper)
dealer.  We built a new hanger with a large door that rose vertically to
open.  The door is counterweighted by two large (5ft X 5ft X 7ft) concrete
blocks.  Cables run up from the door, over a couple of pulleys at the top of
some pillars, and back down to the blocks.  About a year after we finished
the hanger (1963?) there was a loud bang and the earth shook when one of the
cables snapped and dropped its block about 20 feet.  It turned out that the
cable was running over some 4" pulleys.  We replaced them with 10" pulleys,
and there hasn't been another failure in 40 years.
----- Original Message ----- 
From: "Dorairaj Isaac" <dnaisaac at sympatico.ca>
To: "VirtualVairs" <VirtualVairs at corvair.org>
Sent: Wednesday, April 06, 2005 8:34 AM
Subject: <VV> pulleys in tunnel


> Hi,
> I would like to know about the pulleys for the emergency cable and clutch
> cable in the tunnel under the car. Why is one pulley larger than the
other?
> Will it be harmful if I replaced the large pulley with a smaller diameter
> pulley?
> Thanks for any information.
>
> Dorai <><
>
>
>


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