The MGA With An Attitude

On 10/8/2013, Bruce Miller wrote:
"On the MGB LH overdrive, mine quit on a ride today after working the first two engagements. I found the overdrive was hot to the touch. Is this normal"?

In general, a gearbox can run moderately hot after a prolonged drive at speed. The heat comes from friction in the moving parts and shearing of the oil film between the gear teeth and within bearings. It is the efficiency loss in the mechanism. 30-HP input and 25-HP output means 5-HP of heat generation in the gearbox. Driving at high speed with high power through put, and driving in the lower gears, will generate more heat. The gearbox can become hot enough to be uncomfortable to touch. Gearbox oil temperature of 150dF or higher (similar to engine oil temperature) is not uncommon. For some race cars running hard the gearbox might be hot enough to turn water to steam on contact (over 212dF). Temperature that high is not good, and it may merit using a gearbox oil cooler or special ducted air for cooling the gearbox.

The overdrive unit can be a contributing factor to heat generation. In direct drive (overdrive off) the OD unit is just idling along an a couple of roller bearings. When OD is engaged, there is power going through a planetary gear set that will generate some additional heat.

There are two cone clutches in the OD unit, one engaging direct drive (OD off) and one to drive the planetary gear set (OD on). These cone clutches when engaged have no relative motion between the friction surfaces, so they generate no heat. When the OD unit is in the process of shifting in or out of OD, there is some very short term slippage of the cone clutches then will generate a small amount of heat, but only for a second or so with each shift while the clutch parts are slipping before the speed matches. Once the clutch is fully engaged, no slipping and no more heat.

The OD cone clutch is hydraulicly operated via an internal oil pump. The oil flow generates a little bit of heat, mostly due to high pressure oil flowing over a pressure relief valve to return to the sump with zero pressure. Power used to drive the pump is turned into waste heat as there is no useful output work being done. In hydraulics, pressure drop times flow rate equals work. When no output work is done, the related heat energy can be calculated.

The big heat kicker can come when something is wrong with the OD operation. If there is insufficient hydraulic pressure the cone clutch might slip profusely, either continuously or intermittently. Slipping cone clutch under load with power throughput can generate a lot of heat, which could result in a scorching hot housing, or even boiling oil (all very bad). To make any judgment about this you need a temperature probe on the gear case or in the sump oil to quantify how hot is "hot". The OD unit shares common oil sump with the main gearbox, so if the OD unit is generating excess heat the whole gearbox will get almost equally hot. If you can determine that the OD unit is significantly hotter than the main gear case, that would imply that the OD unit is generating an unusual amount of excess heat.

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