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From: (Andy Dingley)
Subject: Re: Hypoid vs. Non-Hypoid
Date: Tue, 09 Jan 1996 17:45:49 GMT

Mark <74551.2327@CompuServe.COM> wrote:

>Can someone explain the difference between Hypoid and Non-Hypoid 

"Hypoid" is not really a question of oil, so much as a question of
gearcutting. Old (1920's) rear axles used straight bevel gears to form
the crownwheel and pinion. These had two disadvantage, the pinion
shaft meets the crownwheel on its central axis, and the straight cut
gears are noisy. By using a more complex "hypoid" gear tooth shape (if
you look at a pinion, the teeth appear twisted) these problems can be
addressed. The more gradual engagement of the teeth along their length
reduces noise. By careful design of the geometry the pinion can be
made to mesh _below_ the axis of the crownwheel. As the centre height
of the crownwheel is fixed by the wheel height, this allows the
propshaft to be lowered relative to the car body, giving a clearer
floorpan and lower centre of gravity for better cornering. Hypoid
bevels are now universal in this application.

Because of the sliding contact that hypoid gears make, their
hydrodynamic contact pressure is higher. To be suitable for use with
hypoid gears, a lubricant must be capable of resisting high pressures.

Oils with "EP" ratings (Extreme Pressure) such as EP90  are required.
Some brands describe themselves as "hypoid" instead, a term which is
synonymous with EP. GL-5 is a formal API standard for this type of oil
(comparable to MIL-L-2105B/C/D)

> The book is telling me to use Non-Hypoid gear oil 80W or 
>80W/90 on the manual transmission and GL-5 hypoid gear oil 90W on 
>the rear axle.  

A manual transmission won't usually contain hypoid gears, so it
doesn't need an EP oil. Rare exceptions are those transaxles where the
crownwheel and gearbox share the same lubricant. Although an EP oil is
more complex to manufacture, it has no disadvantages when used in
instances where the EP attribute isn't strictly required. Manual
steering boxes and other slow-moving oil-containing components are
often filled with 90 weight oil. It's usual to buy EP90 because that's
what the axle requires, then use the same oil for all other

There's little practical difference between 80 & 90 weights. I fill
everything with EP80 and I've never had a problem.

There's an increasing trend amongst manufacturers to reduce the number
of different lubricant types required. My own gearbox (5 speed Range
Rover) runs on ATF, but 20W/50 engine oil or EP90 axle oil are equally

Andy Dingley                      

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...and all most people want to do is pick their noses with them.

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