Gear Lubricants

by Eric Worpe

This article first appeared in the Octagon Bulletin. Eric has since looked at it again and has updated it.

If you’re not confused about the various types of transmission oils available, then you may not have understood the problem. This may seem a provocative aphorism, but it could just be the case.

One of the great advancements of gear teeth design was a tooth profile that enabled gear teeth to roll over each other and still maintain constant velocity. This design reduced friction as it eliminated any sliding action at the narrow points where the teeth make contact.

Many early cars started off with straight-cut gears in the gearbox and bevel gears in the back axle, if you were lucky. However, the limited knowledge in material science necessitated a hefty approach to construction if failure was to be avoided, resulting in low tooth contact pressures and modest demands on lubrication as power outputs were equally modest.

However, although straight cut gears are efficient, they are also noisy due to the interrupted progression from one tooth to the next. The quest for quietness and a higher rating capacity led to helically cut gears such as the spiral bevel gears found in differentials. These overcame the interrupted tooth engagement problem by enabling at least two teeth to be meshed at any one time, directly increasing the strength of the gears. This consideration and the improvements in metallurgy, resulted in both smaller gearboxes and crown wheels & pinions, placing even greater demands on the film strengths of lubricating oils at the tooth contact points.

Helically cut gear teeth and spiral bevel gears are less efficient partly due to the degree of “sliding” that is inherent in helically meshing teeth and consequently generate more heat at the tooth contact areas. This situation combined with increasing tooth contact pressures from rising power levels and the diminishing size of gears, could result in the breakdown of the oil film in the contact area. One surprising effect from an oil film breakdown is the potential transient “micro-welding” together of the contact surfaces; such areas show up as pitting or spalling and can result in failure of the hardened teeth surface.

Film strength additives were introduced to resist rupture of the oil film, whilst high pressure additives prevented welding together of mating surfaces should the oil film break down. ZDDP (zincdialkyldithiophosphate) has become well known as a film strength additive, whilst high pressure additives do seem to have some notoriety. High pressure additives work by forming a thin film on the mating surfaces due to the intense local frictional heat produced as the gear teeth mesh together. One of these additives is based on sulphur, and has the characteristic smell associated with cat’s pee. Esso used to call their transmission oils EXPEE. I wonder if the wag who thought that one up “blew” his career.

Sulphur reacts with steel at high temperatures to form a tough film of iron sulphide, which prevents welding and scuffing taking place. However, sulphur is rather corrosive, so substances such as sulphurised fatty oils are preferred. High pressure additives are of two types, active and mild. The active types contain compounds of sulphur or chlorine and should be avoided when brasses or some bronzes are present, because in the presence of water they tend to form sulphurous and hydrochloric acids, which attack any zinc alloy. Other, less aggressive additives are often based on compounds of phosphorous or metallic soaps.

Initially, transmission oils for helical cut and spiral bevel automobile gears were known as High Pressure oils, such as Castrol HI-Press; both the levels and aggressiveness of the additives were modest and thus suitable for use with “yellow metals” such as bushes, selector forks and the synchromesh cones machined from brass or bronze.

Spiral bevel pinion teeth used in the differential are heavily loaded, particularly the high ratios above 4:1. A variant on spiral bevel gears called hypoid was introduced in 1926 by the Gleason Company in the USA. The centre line of the pinion was offset and this gave increased strength to the pinion’s teeth, owing to its greater tooth contact length; however, it also increased the sliding component in addition to the normal helical meshing of the teeth. This placed even higher demands on the transmission oil and increased levels of additives were used to cope with the higher tooth contact temperatures. To help differentiate from previous high pressure oils, these oils were called EP for Extreme Pressure and due to the more aggressive additives, any “yellow” metal components containing zinc had to be avoided.

So initially, we had Hi-Press oils for gearboxes and spiral bevel differentials and EP oils for Hypoid differentials. Such a simple system was then confused by labelling all these gear oils as EP. Differentiating between the oils was partly left to the description on the container, e.g., Castrol Manual EP80W or Castrol Differential EPX80W/90, Shell Spirax 140EP (not for hypoid axles) or Comma Hypoy EP90 gear oil. Additionally, all gear oils should also be specified using the American Petroleum Industry (API) GL (gear lubricant) ratings.

High Pressure oils suitable for gearboxes and spiral bevel gears are designated as GL-4, whilst Extreme Pressure oils for Hypoid gears are GL-5. For Hypoid gears exposed to very heavy duty, the use of an API GL-5, 75W-140 oil may be needed as opposed to the normal GL-5, 80W/90. Halfords manage to label their own brand fairly sensibly and offer “Gear oil EP80W/90 GL-4” or “Differential oil EP80W/90 GL-5”, “differential” in this case being the more common hypoid.

To simplify lubrication of some cars, just one type of oil, e.g., Millers Hypoid 90 may be specified for not only the hypoid differential, but the gearbox and steering box as well, despite the helical gears in gearboxes not needing oils to GL-5 specs. Some gearboxes are even supposed to be able to run on engine oils that contain high levels of ZDDP; this may seem strange, but API viscosity ratings for gear oils are not directly comparable with those for engine oils, A 75W/90 gear oil has an equivalent viscosity to a 10W/40 engine oil. Any deterioration from using unsuitable lubrication is not likely to be apparent until it’s too late, as only a close inspection of the gear teeth would reveal developing problems. So, beware of the line “I’ve been using “XXX” for yonks, and it’s fine”.

To sum up, early bulky straight cut gears with low tooth contact pressures were adequately protected by oils with minimum levels of additives, such as those designated GL-1 to 2. The introduction of helically cut gears and improved materials resulted in designs with higher tooth contact pressures, which in industry are protected by oils with API ratings from GL-2 to GL-5. For automobile oils, GL-4 and GL-5 gear oils seem to be universally available. GL-4 is suitable for most gearboxes and spiral bevel differentials and has modest levels of high-pressure additives, which do not attack “yellow” metals. GL-5 oil is suitable for hypoid gears, where extreme pressure additives are needed, and should not be allowed in contact with “yellow metals” unless the specifications say otherwise.

Other brands are available.