Various Fluids as Gear Lubricants

While a  number of  viscous fluids have no  doubt  been used  as  gear lubricants, most of these, other than the  petroleum or synthetic  oils, are  deficient in  desirable  characteristics. However, glycerol has been   suggested as a lubricant and carrier for molybdenum   disulfide. The  specific  application in this case  was  on  small gear sets of  the  worm  and  hypoid  types. Also, molasses was used as a transmission gear lubricant in France during World War II.

The London Transport Company has experimented with   an inhibited castor oil in the axles of certain buses. The thought behind this use was to reduce the fuel consumption of the vehicles. Low viscosity oils or  synthetic fluids were  previously tried but  with  such  lubricants the  necessary  damping  effect was  absent so that  proper shifting was  not  possible. Caution  is given that  this  application of  castor oil is only  possible  under  the  stop and go  operation of busses and  would not  be possible  in over the  road  vehicles. The axle in this case consists of a worm drive with a bronze worm wheel.

Gear pumps depend upon the fluid being handled as the lubricants, and this is sometimes water. Also, water has been employed as lubricant in the case of some nonmetallic gears but has its limitations. First, the temperature range over which water can be used is limited. Next, it will contribute to rusting of the ferrous parts with which it comes in contact unless inhibitors are present. Further, water affords little protection against wear.

Spray or Jet Application of Gear Oils

Fluid lubricants can be applied to gear sets by sprays, which are air borne, or by jets of the liquid alone. Such methods of application are employed when the peripheral speed exceeds the limits for splash lubrication or when the gears are not adapted to the dip method. The size and shape of spray patterns can be controlled by the use of different type of nozzles, the distance from the nozzle to the target, or the pressure and temperature of the lubricant. If the gearing has a wide face, two or more jets can be used on one gear set.

 Since one function of the gear oil is to cool as well as lubricate, a fan shaped spray or jet should spread the oil across the entire gear face. Adjustments of such sprays are a matter of experience, and firms supplying such equipment can give the best advice. For any cases, except exceedingly high speed, most authorities prefer to apply the oil spray on the incoming side of the gears so that the oil will be spread across the entire gear face. For moderate speeds the application should not be directly at the point of mesh, due to air turbulence. However, Dern^15 suggests that, for pitch line velocities above 16,000 to 18,000 feet per minute, the most satisfactory results are obtained  by spraying a solid stream of oil radially into the teeth of both gears at a points  as close as possible to the mesh. Further, in speaking of gears operating at pitch line velocities of 20,000 to 25,000 feet  per minute, Dern^15 agrees that  such  gears should be lubricated  on the leaving side of the mesh  where  the oil will  do the most  good as  a  coolant and not contribute to high dynamic loads.

In high speed operation of gears, lubricated by jets of oil, scavenging of the lubricant is sometimes a problem. Special  baffles or even a  change  in gear  case  design  may  be necessary  in order  to remove  considerable  volumes of oil  rapidly  enough  so  that  interference  with  gear  operation  does  not  result.

Automatic  spray  application  is used  on open gears where a controlled  amount  of  lubricant is  sprayed  over the gears at  desired  intervals. By heating  the oil, high  viscosity  lubricants can be  used  or  better  still,  cut  back oils  are applied. Gesdorf^21 offers details of such applications and the problems involved in the development and use of the same.

For  high speed  worm gears, which tend  to throw  oil  rather than  carry it to the  mesh, a  high velocity  jet stream  of  gear oil  is more  satisfactory  than  a  spray. In fact  two jets can be  used, one striking the worm  wheel  teeth  in a direction  parallel  to  the axis  and  the  other in a  direction  perpendicular  to  the  axis.

Types of gears to be lubricated

Where   gears  are  on  parallel  axes, either  spur  or  helical  gears  are  generally  employed. Either type can be used as external or internal drives.   The  herringbone  gear  is  similar  to  two  helical   gears  having  reversed   directions   of   spiral, placed  side  by   side  so  that  the  teeth  come  together   to  form  a  chevron   pattern. The rack and pinion, used to convert rotary motion to reciprocating, generally   uses a spur gear.

For   intersecting   axes either straight   bevel    or   spiral   bevel   gears   are used   as a  rule. The   latter type may be used   on angle   drives where   the   shafts    do not    intersect at full   90 degrees. The contact   of the   teeth in such gears gives   a rolling motion. With  non intersecting  and   nonparallel  axes  the  types   of   gears  used  are  crossed   helical, single  enveloping  worm, double  enveloping    worm, or  hypoid. Here the  contact  of  the  teeth  gives  a  sliding  as  well  as  a  rolling   motion. In  most  cases  a  gear  set  will  be  used  to  change  speed, and  in  such  cases  the  smaller   gear  is  designated  as  the pinion. Both  the  number   of  teeth  on a  pinion  and  the  ratio    of  the  teeth  on  the  driving   and  driven   member   may  vary ,   but  with  bevel  gears  there  is  seldom  less  than  12 teeth  to  a  pinion.

While   some  spur  and  straight  bevel  gears  are  still   made  of  cast  iron, the  tendency  in  all   types   of  gearing  is  for  the  use of  steel. Exceptions   will  be found  to  such  practice, for  example,  in the   use  of  bronze  for   one  member   of  worm   gears. Some small gears and even   larger pinions are   made   of plastics, such as ‘Delrin,’’ ‘Nylon.’’ ‘Teflon,’’ etc. Pinions  have been and   may still be  made  of  rawhide, pressed  paper,  etc, but  our  concern  is  primarily   with   lubrication  of  metal  gears.