re: "... The health of the 8 springs that keep the unit in direct drive
is important to keep the clutch from slipping...."
I believe the shop manual admonishes owners to replace these when
overhauling the unit, even if the old ones look and feel fine (some say
not so). I think my old ones are in a box on a shelf somewhere if
anybody wants them.
Note the O/D pump is a positive displacement pump that will continue to
produce pressure--driven by the full power of the engine--unless there
is a pressure relief mechanism.
On 8/11/2023 10:34 AM, Hank Leach via Healeys wrote:
> Guys-this is a very lively topic and one of sometimes great mystery
> and confusion. The unit is complex but the end result is working well
> or not functioning at all.
>
> On the subject of pressure required to operate the shift into
> overdrive there are many schools of thought. Some people replace or
> stretch springs or add washers to increase the pressure needed. The
> health of the 8 springs that keep the unit in direct drive is
> important to keep the clutch from slipping. The pressure supplied to
> overcome the strength of these 8 spring , and effect overdrive, relies
> on adequate hydraulic pressure. The change from direct to overdrive
> needs to be smooth, not harsh, and if pressures are high the unit will
> snap into and out of OD rather than the smooth interchange we are used
> to while driving.
>
> When applied to the Austin Healey 100, the Laycock overdrive was
> initially a 32% reduction unit. It was soon found out in these early
> Warwick cars that 32% was too much reduction. DHMC swapped units
> three times in the original DMH tour car until they finally found a
> combination that produced the smooth shift and complimentary gearing
> desired.
>
> It is difficult, today, to find any information on this original 32%
> gearbox, however if you examine the booklet (attached) supplied by
> Laycock for the early cars, you will notice that the casting allowed
> for a large accumulator piston (seen on p6 in brochure). After some
> research and modified engineering, the company came up with this alloy
> "sleeve" called the piston housing (photo) which partially filled the
> chamber of the original large accumulator casting and allowed for a
> smaller accumulator piston that we see in all the later 28% reduction
> units. That size piston, in proper condition, produced from 350-450
> lbs of needed hydraulic pressure to overcome the springs influence.
>
> I'm confident that the remark made by Geoff Healey in his writings
> about the early transmissions "harsh shifting" revolved around this
> huge accumulator piston producing too much pressure. Additionally,
> there were three various accumulator springs supplied which adjusted
> the pressure. The early boxes produced about 350lbs and the later
> about 450.
>
> Today, Overdrive Spares supplies that sleeve housing with only one
> rubber O-ring as it is basically a filler part and not under extreme
> pressure. The accumulator piston within is under that pressure. They
> supply a modified piston using O-ring technology. So, it is not
> unusual to get good pressures over the normal 450 lbs required. If it
> works well don't fix it. The only concern is if the pressure does not
> release when overdrive is not called for. A clean unit, with proper
> 30W oil, will allow that to happen. All this some old theory and
> history to ponder. Hank
>
_______________________________________________
Support Team.Net http://www.team.net/donate.html
Suggested annual donation $12.75
Archive: http://www.team.net/pipermail/healeys
http://autox.team.net/archive/healeys
Healeys@autox.team.net
http://autox.team.net/mailman/listinfo/healeys
Unsubscribe/Manage:
http://autox.team.net/mailman/options/healeys/mharc@autox.team.net
|