Load carrying Capacity SAE Test Apparatus

As previously mentioned, this machine is a device in which two Timken test cups, no. T-48651, are rotated  in line  contact  with  each  other  and  in  opposite  directions, with  provision  for  controlling  the  speed  of rotation, the  slipping  velocity  and  the  rate  of  applying  pressure  at the  line  contact  between  the  rotating  cylinders  or  cups. To obtain reproducible  result, it is  essential  that  the  surface  finish  of the  cups  used  be  uniform  and  that  the  shafts  on  which these  test  pieces  are  carried be in  perfect  alignment. Both points are covered in details of the test methods. The  total  variation  in surface  finish  shall  not  exceed 10  micro inches and  the  inside  and  outside  surfaces  of the  test  cups  shall be  concentric  within  0.0005 inch.

With the  lubricant  tester  in good  mechanical  condition, test  shafts  true, and  the  alignment properly  adjusted, the  test  cups of  specified  surface  finish  and  concentrically  are  placed  on the upper and lower  shafts. With the oil box overflowing, an initial load of 15 to 20 pounds is applied. The  machine  is  then  started and  at the  end of a  30 second  period   the  automatic  loading  device, at  the desired  rate  of loading , is  started. The test cups are then observed for signs of scoring. This  is most  readily  detected  by  observing  the  lower  test  cup  on the  trailing  side  at a  position  approximately ½  inch  from  the  contact  line. When  signs of  scoring  are  detected, the  drive  motors  are  stopped  and the load  removed. After  thorough  cleaning  and  with  new  test  pieces and  fresh  oil  the test  can  be  repeated.

The suggested conditions of test are a main shaft speed of 1000 rpm and a rubbing ratio of 14:6:1. The  load should  not exceed 450 pounds and  if no  scoring  occurs  at this point  the test  is stopped to  prevent  over heating  of the  shafts, etc.

This test  has been  primarily  used for evaluation of automotive  gear  lubricants  but has not  replaced  tests  with gears.    

                                                                                                                                                                                                              

Desirable low and high temperature qualities of gear oils

Transmission and gear oils which are used either in vehicles or in outside locations in cold climates should remain fluid at low temperatures. Low temperature is a relative term and an extreme case is that of aircraft flying at high altitudes which, therefore, encounter temperatures approaching – 70 degree (F). The gear oils used in this case are compounded from synthetic fluids.

Disregarding synthetic lubricating fluids, two general types of mineral oils are available. These are naphthenic and paraffin. The former, as a rule has the best cold test because no wax is present, but as temperatures are reduced such oils may tend to become quite viscous. This tendency will vary both with the viscosity and the source of the base oil. Most paraffin type oils contain wax which crystallizes at low temperatures and may therefore form a solid mass which will not flow or pump and may even channel in a gear case so that the gear teeth would lack a lubricant. By the addition of pour depressants, generally certain  types of  polymers, the wax crystals are coated  so  that in place  of growing to long  needles  thy deposit as fine crystals which produce a slurry  which moves at a lower temperature  than the untreated oil.

A more positive approach to securing low cold test gear oil is to choose lubricating oil which has been refined so as to have a low pour point. Low viscosity oils generally have lower cold tests than high viscosity oils of the same type. Thus, lubricating oils are available with viscosity of 70 to 80 SUS at 100 degree (F) which have pour points of -60 to -70 degree (F). Proper dewaxing will also lower pour points. In extreme cases gear oils are offered which consist of lubricating oils diluted with fractions boiling below the lubricating fractions. Likewise, in the arctic regions Kerosene has been used to dilute gear oils. This poses a  hazard to the equipment  if the diluted  lubricant  is not removed  from  the  gear  case  with the advent  of normal temperatures.

High temperature limitations of most gear oils will be due to deterioration of the oil and additives. Oils should be chosen having high flash points if operating temperatures are excessive. The flash point of oil is generally indicative of the temperature at which volatility starts. In most industrial gear applications, the bulk oil temperature seldom exceeds 125 to 150 degree (F). In vehicles such temperatures seldom exceed 300 degree (F). Therefore, for  what considered normal  gear lubrication, additives are  selected  with  the  thought  that  temperatures of  use  will  not exceed 300 degree(F)

However, Borsoff et al ^8 describes tests run at both 400 degree (F) and 600 degree(F) on a  gear test  machine in which the gear compartment and the oil were  heated while  the machine  was in operation. Conclusions drawn from this investigation were:

                                   “Due to the thermal instability of the lubricants, the operation of gears for any prolonged period of time at 600 degree (F) or higher is inadvisable; (2) no new or unusual types of failures were observed during the operations of gears at 400degree (F) and 600degree (F). Just as at normal operating temperatures, scoring and abrasion were the two primary destructive failures; (3) the load carrying capacities of oils decreased with the increase in temperature. However, at temperatures over 400degree (F) gum deposits are formed by oils. These deposits serve as a protective coating and improve scoring performance of gears and gear lubricants”. For this reason SAE 20 and SAE 30 mineral oils and “Plexol 201” showed about the same load carrying capacity at 400degree (F) as at 100 degree (F). However, 1010 grade and SAE 60 mineral oils, “Ucons LB-170” and 50-HB-170 and “DC-200” silicone fluid all scored at lower loads at 400degree (F) than at 100 degree (F).

Bowden and Tabor^11 investigated the frictional behavior of oil films on steel surfaces as they were heated and as oxidation took place. At 302 degree (F) after 30 minutes heating the friction became low. At 392 degree (F) the reduction in friction occurred after 15 minutes heating and at 572 degree (F) low friction was observed after less than 2 minutes heating. In the latter case if heating was continued for 20 minutes a thick gum was formed and  friction increased to a high value. The above action was no doubt due to the formation of acidic products due to oxidation. Such products, in time, might lead to corrosive wear. Next the oil would increase in viscosity and compounds would from which are insoluble in the oil. When and if acidic products result from heating of gear oils, metal compounds will eventually from and these in turn will act as catalysts to promote further oxidation.

In view of the fact that high temperatures cause rapid deterioration of most gear lubricants, equipment should be so designed that operating gears are subjected to only moderate temperatures if long life of gear oils is expected.

However, since the military  desire gear lubricants for  jet aircraft  and  other agencies for space vehicles which  will stand  very  high  temperatures, it can be  expected that synthetic fluids or solids will  be  available  which will withstand 700 degree (F) or even 1000 degree(F) for a period.

Dissipation of heat by gear lubricants

Under the most ideal conditions of lubrication of two moving metal surfaces heat is developed. In fact Bowden and Tabor^11 found that, even though lubricating films are present, surface temperatures of metals  may exceed several hundred degrees Centigrade  at  relatively small loads and sliding speeds. Blok^6, 7 first postulated and then verified conditions of “temperature flashes” between operating gear teeth. The temperature at the points of contact was shown to be proportional to CfP^3V, where C is a constant, f is the coefficient of friction, P the mean pressure, and V the gear engaging speed. This formula holds for both spur and hypoid gears, but the action of the latter type develops the greater amount of frictional heat. Since the contact points are small with respect to the overall dimensions of the gears, this heat is conducted into the two moving metal surfaces. A lesser amount of heat may also be developed by churning friction where gears are bath lubricated. 

Gear oils are an aid in dissipating this frictional heat. How effective this action is depends upon the amount of fluid coming in contact with the gears as well as the temperature and viscosity of the oil and the manner in which such oil is flushed over the gear teeth. Oils are not the ideal coolants since the specific heat of petroleum products is about half that of water.

Design and application influences heat dissipation in that the size of the gear case determines the total gear oil present and radiation from the fluid and the metal depends upon the surface exposed. If the oil application is by spray, the jets can directed at the points where the greatest heat is present, perhaps on the leaving side of the gear teeth. Circulating systems permit not only placement of oil streams but also adjustment of quantity. In case heat dissipation is not rapid enough, additional oil storage or settling tanks can be used to provide more radiation.

The lower the viscosity of  the lubricant the more effective it is in transferring heat from the tooth surfaces to the bulk oil and then to the gear housing and thence to the atmosphere. The value of low viscosity gear oil in dissipating heat was shown in certain truck operations. Here the differential oil ran about 35degree ( F) lower  in  temperature  when an SAE 90  lubricant was  substituted for an SAE 140 gear oil.