[Healeys] Question for Shop Owners / Restorers Only - OD

[Richard Ewald]

On Tue, Jul 15, 2008 at 8:40 PM, Richard Ewald <richard.ewald at gmail.com>
wrote:

> This doesn't seem to make a lot of sense to me.  Negative torque?  I think
> you misunderstand the purpose of the one-way clutch (sprague clutch if your
> prefer)  The one way clutch will rotate freely in one direction, and lock up
> in the other.  Just like a 3/8" hand ratchet.  You can hold the socket
> stationary and turn the handle, or hold the handle stationary and turn the
> socket, neither action will harm the ratchet.
> So why a one way clutch?   The reason for it is because the surface area of
> the cone clutch is very small and is not capable of transmitting the power
> of the engine without slipping.  If you look at the exploded diagram of the
> overdrive you will see that the one way is used in direct drive. without the
> one way clutch you would spin and burn up the cone clutch with the power of
> the engine.
> Here is a link to a design and function document for a later type Laycock
> OD used on a Volvo (basic design of the planetary gear set / one way are
> identical, hydraulics differ)
> http://www.volvoclub.org.uk/faq/TransmissionManual.html#ManualM46LaycockOverdriveUnitBasicOperationYou will find a link on that page, for an exploded diagram.
> For what its worth, the owner's manual for a Volvo 240 Turbo stick shift
> contained a warning that if turbo boost was achieved in reverse (when the
> one way is non-functional) slippage of the cone clutch would occur and this
> was normal!  So putting power to a one way clutch in the wrong direction
> won't hurt anything.  If you think about it, every time you lift off the
> gas, the rear wheels are trying to overrun the engine.  Damage to the one
> way clutch does not occur.
>
> My personal Wild Assed Guess is that the throttle switch is to reduce the
> shock on the rest of the drive line, such as u-joints, trans mount, rear
> axle mounts etc.  By forcing you to open the throttle the revs are brought
> up and there is not as big a shock as the OD disengages.  Also opening the
> throttle would tend to limit the manifold vacuum a bit which would limit the
> amount of oil sucked past the rings and valve guides (something modern cars
> do as a matter of course).  I would also note that throttle switches are not
> used on other cars that used these units.
> Rick (Who used to teach trans and OD rebuilding for Volvo Cars)
>
> On Tue, Jul 15, 2008 at 4:05 PM, Mr. Finespanner <
> mrfinespanner at earthlink.net> wrote:
>
>> Alan,
>> I'm with Rich on this one.  The throttle switch is there to prevent
>> negative torque on the overdrive unit when the OD is switched off.