Morning Nitai

I think the SAE numbers used to specify motor oil are not comparable with the similar SAE numbers used to specify Gear Oil. In practice, SAE10W and SAE70W have similar viscosities:

• SAE10W Motor Oil – 4.1 sq/mm per second at 100C
• SAE70W Gear Oil – 3.9 sq/mm per second at 100C

I think the 'W' stands for Winter and means the specification covers low temperatures, i.e the oil won't thicken unreasonably in cold weather.  (Edit: Hopper posted first, see his comment on the W)

Dave

Edited By SillyOldDuffer on 18/10/2016 12:13:02

So how are we to tell that a gear is a gear oil? The numbers EP at the end of the specification that's how , and therin lies the rub. A gear oil behaves like an oil with a viscosity of 70 , 90 or what ever under conditions of extreme pressure. however when it's not under extreme pressure , then it behaves like an oil of whatever viscocity it's made of , which is not stated. More modern gear oils , usually to GL5 spec are multi graded, many older gear oils are mono grade oils. I had a tin of SAE 140 which went in an old Villiers motorbike , it was incredibly viscous and was like pouring golden syrup. EP90W140 pours like a motor oil.

Universal tractor oil is suitable for engines and gearboxes, but is quite low specced in comparison with modern motor or gear oils.

,The numbers EP at the end of the specification that's how ,

Just not true. EP (extreme pressure) grades are for things like anything but straight spur gears! Most EPs are limited to use where there are no 'yellow' metals present. Go figure!

The 'W' rating really relates to cold oil and the other to normal operating temperatures. Nothing more, as it means that the oil will not congeal (and not pump/circulate) at starting temperatures for an engine, provided the minimum temperature is adhered to.

I doubt that any gear oil has a 'W' rating? It would mean the gears were just lubricated by 'grease'!

I found this in a technical report:

"Each low temperature gear lubricant (SAE grade followed by a W) must not exceed a maximum viscosity of 150,000 cP at the temperature indicated. A 150,000 cP fluid appears like a semifluid grease which will barely pour."

The background to the paragraph states that damage occurs when a Gear Oil becomes more viscous than 150000 cP. As this could occur during winter in a cold country the use of W grades is recommended. In hot countries a W grade oil is unlikely to be needed, though I guess there's no reason why one shouldn't be used.

I can't remember the proper numbers but in aviation work we used Aero Shell W-100, and W-80 these were detergent oils used in aero engines, a caution goes with these oils "DON'T mix with non detergent oil. What happens? Early 1950s RNZAF Hastings Mk 3 transport aircraft at Singapore, refueled, and oil filled by RAF ground crew, filled with straight oil, the result of the mixed oil was that all 4 engines stopped just after take off because the oil foamed on mixing. The RAF used Mk 1 and 2 Hastings with earlier engines, and straight non detergent oil.

Ian S C

I believe it was more for working mechanic shops where they would have a stock in the workshop of 30 weight or 40 or even 50 weight etc engine oil. Their stock of gear oil would have been 90 weight gear oil. So, supposedly, there was less temptation to use gear oil to top up an engine, or engine oil to top up a gear box if supplies of the other ran out. If they more correctly labelled the gear oil as 50 weight or whatever its viscosity really was, it may have ended up in somebody's engine.

I don't know where I know this from, just one of those things floating around in the back of my mind. I think it was done way back early 20th Century when car mechanicing was a new profession.