Truck Transmission and Differential Lubricants

Truck gears, in the main, require higher viscosity oils than do the gears in passenger cars. However, it is difficult to generalize since any one manufacturer of trucks may employ a variety of methods of speed reduction as well as perhaps more than one type of differential. However, lubrication charts are kept up to date and such charts, as well as the recommendations of the truck manufacturer should be consulted as to the proper grade and type of gear lubricant to use. Lubricating charts should be read with care because blanket recommendations are not made for all trucks of a given make. Also footnotes  are  frequently used  to  provide additional  or  more  detailed  information about  truck gear  oils.

A number of truck manufacturers depend upon specialty companies to provide them with transmissions or axles. In this case the parts manufacturer may suggest the most satisfactory gear lubricant. These firms often issue. Field maintenance Manuals devoted to each type of speed reduction unit or each type of axle. Such manuals generally contain sections on lubrication.

Almost all truck distributors suggest some variation in the viscosity of gear oils used as the temperatures change. A blanket statement cannot be made, but sometimes above 32 degree F an  SAE 140  grade of  lubricant  will be  recommended and  below this  an SAE 90.

While it has  been pointed out that lubrication would be  simplified by the use of a common  lubricant  for  transmission and  axles of  trucks,  there is not  an  agreement on this. Thus, Nelson and  Valentine^40 mention that a rear axle  lubricant must be  capable  of  absorbing  greater  torque  or providing  better  load  carrying  qualities  than  a  transmission, particularly  in  the  first  three gears.

Fletcher^19  notes that  many fleets have  standardized on the use of  engine oils in transmissions and  cites  a large  bus line which has used  SAE 30  and  modified SAE 10 engine  oils in  transmissions  for  many  years. The 30  grade is used  in straight mechanical boxes  and  the 10  grade  when mechanical  transmissions are  combined with  a  converter or coupling  and  a common  oil  supply  is  required for  the  two  units. This author objects to the oxidation and sludging of EP oils compared to straight mineral oils. His suggestion is the use of SAE 50 oil for transmissions. Another argument in  favor of  the  engine oils  is that  in  corrosion tests, using bronze specimens, such oils showed  little corrosion whereas  both  HP and  MP  oils  showed excessive corrosion.

On the other hand, Calish^8 cites evidence of the practicality of the use of a common transmission and axle lubricant in heavy equipment. A specific additive oil composition was used in some 1420 vehicles representing sixteen makes of trucks. The inference is that such a uniform transmission and axle lubricant proved satisfactory to the thirty-four accounts operating the above trucks over a period of several years.

When transfer cases are used to drive various devices, the same grade and type of oil as is  used  in the  transmission case will be satisfactory. Transfer  case  is a term applied to gear assemblies which transfer  power from  the  main  drive  line to  auxiliary equipment such  as  front wheel  drives, pulleys, hydraulic pumps, windlasses, and other mechanisms. For example,  many heavy  duty utility  vehicles carry winches for  erecting  heavy poles, devices for  boring holes  in  the  earth, and  other tools which  must  be  driven from  the  same  power source.

Where trucks  are  equipped  with  automatic  transmissions, ATF is  satisfactory  for use  and is so  listed in most  lubrication charts. However,  as a  matter  of  economy, General Motors  suggests the  use  of a  Hydraulic transmission Fluid, type C for  certain heavy duty trucks, buses, and  earth- moving  vehicles. Type C does not  have all  the  characteristics of  ATF  but  does  have  to  pass a  Powerglide Transmission Test, have  low  varnish and  sludge  deposits, and  must have  a  minimum effect  on rubber  seals. This Type C should never be substituted for ATF where the latter is specified.

Truck  Axle Lubricants. There seems to be little question as to the necessity of EP gear oils in axles of heavy duty trucks. However, there is not  common agreement as to  the  proper  grade  or  viscosity  of  lubricant  for  such  applications. Johnson^27, who considered these subjects, states:

“ Our  field experience is quite  conclusive to  the  effect  that  an SAE-140  viscosity lubricant is  far  superior to an SAE-90 in  its  ability  to  prevent gear wear  and  related  problems. On  numerous occasions  we have been  able  to  overcome  gear  wear  problems by  merely  effecting  a change  from  an SAE-90 to an SAE-140 viscosity”.

Also, it was found  that, with the heavier gear oil, temperatures in the  gear  case  did not  increase  but actually were  5 to  10 degree F lower than when  operating  with  the  lower  viscosity oil. The use of SAE 250  gear  oil  was  investigated but  it  was  found  that  this did run hotter  than  the SAE 140 grade.

A  further  advantage  of  the  use  of the higher  viscosity oil was found  by  this  investigator^27 when  the two  grades of hypoid lubricants were tested  on  eighteen driven units in the laboratory, using hypoid gears with a  61/6 ratio. The results obtained were:

“At 3000 rpm, the SAE-140 lubricant supported a torque load 21% greater than the SAE-90. At 2500 rpm, the advantage was 23%, and at 2000 rpm, it was 20%”.

Another factor, indicating the desirability of higher viscosity oils in axles of trucks, is that pointed out by Blok^3. It  was  mentioned that,  under  conditions of  impact  load  on  gears, viscosity is of  prime importance and  that  this  property of the oil  cannot  be  replaced by  antiwelding  activity  if wear  is to be  prevented.

Gears in  heavy duty trucks are  subjected to  low  speeds  and  high  torque conditions during much  of  their operation. For example, Nelson and  Valentine^40 determined that  in  highway operation a truck having  a 5-speed transmission used first speed about 2 per cent of the time, second  speed about 3 per cent, and  third speed  about 10 per cent , or a total of  15 per cent. The resulting comment was: “It is in this interval that gear destruction is prevalent, if the lubricant does not have proper EP qualities”. Further, mention is  made  that  in  off -high way  service, such trucks  operate approximately 50 per cent of the  time  in  the  first  three transmission gears.

Not all EP gear oils will provide satisfactory protection to axle gears operating at low speeds, and high torque, as do those in trucks. Therefore,  assurance  should be  obtained that  lubricants satisfactory for  such service  are  used  for  truck  axles. Lead soap active sulfur EP lubricants have been favored by some for such application. Since the  prevalent operation of  passenger cars is  under  high  speed  and  low  torque conditions, it is  desirable that a single  type  gear  oil  satisfy these conditions as well as  those  of  operating trucks. With this in mind, what is considered by some as a truly multipurpose gear lubricant has been provided? Such oils are those  conforming  to  specification MIL-L-2105B  which  is accepted  for  the  requirements of  all  vehicles operated by  the  U.S Military and by  many  other  users.

Some axle manufacturers and truck producers prefer to  either issue specifications for  what  they  know to be  satisfactory  axle lubricants or  to  state the type of EP  gear oil  to  employ. Hence, it is well to abide by the opinion of the truck supplier when choosing lubricants for truck axles.

This is all the more true because the gearing in truck axles may vary. Most  of  such variations, with  the exception of worm  drives, will probably  require  the  same  type  and  grade of  gear  oil as will  straight  hypoid  gears.  Thus, Rockwell  standard for  either  Planetary Axles or Hypoid- Helical  Two- Speed Double-Reduction Drive Units, as  well as  Spiral Bevel Gears,  recommend oils conforming to  their  Standard 0-65, which  is  an  SAE 140  EP  product. Use of Standard 0-65, the SAE 90 grade is optional but is suggested for lower operating temperatures than the 0-65.

Lubrication of Worm Drives on Trucks. In the case of worm drives on trucks, recommendations for the lubricant vary. While some suppliers of EP gear lubricants claim such oils can be used safely in worm drives, Watson^51 states:

“Actually, in practice, an EP lubricant is usually unnecessary for a worm gear. When lubrication failure occurs in a  rear-axle worm gear, only  in  exceptional  circumstances can it be  attributed  to  lack  of load carrying capacity  of the  oil used”.

Certainly  if temperatures  exceed 200 degree F, as they  may  at  times in axles of any  large  vehicle, EP additives may  cause  excessive corrosion of  bronze. Since EP agents and combinations of the same cover such a wide range of chemical compounds, the above statement may require modification. Perhaps the worst offenders are sulfur compounds, even some not considered active at 200 degree F. The possible  explanation may  be  that since  copper has  considerable affinity  for  sulfur, temperatures at  contact  points  of  the  gear  and worm  will  be  high  enough  to  cause  reduction of the  metal and sulfur. That, is the action may be akin to temperature flashes which occur with hypoid gears.

However, substitutions  have been  made for  recommended worm gear  lubricants; and if this  is done, fleet operators  are in a  better position to  observe result  than are  individual vehicle operators. In  certain  busses of  the  London Transport Company  having worm  drives, it  was  reasoned that  a fuel saving would be possible if there was less  drag due  to  the  axle lubricant. An  exceptionally thin oil  gave  satisfactory lubrication, but  when  changing from  first  to second gear  on  a level road, the  lay shaft  spun so  freely  that  the  higher gear  could  not  be  engaged until the  bus  came  to rest. An inhibited castor oil was then tried and proved satisfactory. However, the operation of these  specific  vehicles with  considerable stop  and start does  not  result  in  heating of the axle  lubricant, with  a consequent  possibility of oxidation, as would be the  case  in over-the –road vehicles.

While possible variations in oils for servicing worm gears of trucks have been mentioned, this is for information only. The safest  course  for  the  owner or  user  of  such a truck  is to follow the  recommendations of  the  manufacturer  of the  equipment.