Nonferrous Metal Rolling Gear Lubrication

Metals such  as aluminum, brass, copper etc., are  rolled  into  sheets or  other  shapes in  continuous rolling  mills designed  somewhat like those  found in steel  mills. Modern mills use circulating oils to lubricate the gear drives, pinion stands, and journal bearings. Either mild EP or MP gear oils of a noncorrosive nature can   be employed in such service. The  grade  of  oil  will depend  upon the speeds  of the gears  and  may  vary  from an  SAE  80  to an SAE  140. In  screw down  equipment  either of the above  type  of oils  or a straight  mineral  oil  can  be  used. Where  any  of the above  drives  are  through open gears, a residual  type  of gear  oil  containing  a  rust  inhibitor  and having  a viscosity  of 1000 to  2000 SUS at 210  degree F  should  prove  satisfactory. The  best  practice  is to  apply  such a lubricant automatically so as  to  insure a coating on  the  gears  at all  times. 

Reduction of friction in gear operation

Bowden  and  Tabor^11  have shown  that  the coefficient  of  friction  for  one lubricated  surface  moving relative  to  another  will  depend  upon  the material  used  in the two  surfaces, the type  and  velocity  of  motion between  the  surfaces,  and  the composition  of  the lubricant. The  first  two factors  are fixed  by  machine  design, but the gear  lubricant  can be altered so  as to aid in providing  desirable  friction  characteristics. The  contribution  of  this  last factor  is  evident  if  we  consider  that the  coefficient  of  friction  of lubrication  metal  surfaces  is  about  1.0  while in  the  case of  such surfaces, lubricated  with  a  boundary  film,  the value  is  about 0.05  to 0.15.

Any explanation of friction is based on the fact that there are irregularities in surfaces. As one such surface moves relative to the other there is an interlocking of the asperities of the two areas. The role of the lubricant in overcoming boundary friction may be partly by filling in the low spots and in the case of a thick film, of lifting one surface over the other. However, under the best conditions, there will be some interlocking of the high spots. If, in so doing, some asperities plow through those of the other surface this will cause one type of friction. The remainder of  the friction  may be a result  of  welding  and  shearing  of  minute junctions between  the two  surfaces.

We are concerned, in the main, with two steel surfaces in rolling or sliding contact. While the thought has been advanced that rolling friction is essentially independent of the lubricant used, gears in operation, particularly hypoid or worm gears, have both a rolling and sliding motion. An example of the variation of friction with the type of motion concerns first a steel rider sliding across a lubricated plate of the same steel to give a coefficient of friction of about 0.14. With the same combination of materials, the rolling friction coefficient was about 0.00015.

Other operating variables to be considered are load, speed, and temperature. Rounds^45 using a test machine in which ball bearings had both a rolling and sliding action and, thus, simulated the action of gear  teeth, concluded that, in general, the coefficient of friction decreases as either the oil  temperature, the ball velocity, or the load  increases. However, the magnitude of the oil temperature and ball velocity effects tend to decrease as the load increases.

In choosing a gear oil one is faced with a compromise because the lowest viscosity oils have the least internal friction and, yet, high viscosity oils will maintain the most satisfactory film to prevent metal to metal contact, particularly at low speeds. This is true when hydrodynamic or thick film lubrication conditions prevail and where viscosity is the most important property affecting friction. While a condition of low friction is desirable in a lubricated gear set, it must be kept in mind that there is not necessarily a correlation between friction and wear. Beeck^3 has suggested that this may be due to the fact  that while wear takes place momentarily  at isolated spots, friction is ordinarily measured as an average of a large area and a longer time interval.

Irrespective of viscosity, various types of mineral lubricating oils may give different friction values. Rounds^45 found that naphthenic base oils gave higher friction values than paraffinic base oils, even after limited attempts to change the friction properties by fractionation or additional refining. In this investigation, the most commonly used synthetic fluids gave friction values similar to straight mineral oils. Since paraffinic oils show less increase in viscosity with pressure than do naphthenic type oils, this may account for the lower friction values of the former type of oil when used in lubricants. The discussion above has been concerned with kinetic friction of fluids containing no additives. When boundary lubrication conditions exist, nonreactive gear oils will not suffice; therefore, consideration must be given to lubricants containing additives, Rounds,^45 using a  naphthenic type oil as a base, found that different additives  would  lower kinetic friction and/or raise or lower static friction. Fatty acids and related compounds were the most effective of the agents investigated for lowering static friction. To lower kinetic friction at velocities above 100 fpm, reactive chlorine and sulfur compounds were most effective.

However, the type of service which demands the use of EP gear lubricants nullifies reduction in friction. For example, Bisson et al.,^5  when investigating the effect of  chemical reactivity of  lubricant additives on friction and surface welding at high sliding  velocities, found some decrease in friction up  to about 1300 fpm, when using  p-dichlorobenzene, followed by  a  very  abrupt increase in the coefficient  of  kinetic  friction. Thus it  was  found that :For all  additives, the existence  of  a  critical  sliding  velocity where  the friction  coefficient  increased  and surface welding occurred  was verified.

Kinetic friction in fluids causes heating and thus power losses. The  friction  due to gear oils  themselves  may be film friction, which  has  been considered, or churning  friction  which occurs  as  the  gear  teeth  rotate  in the oil bath. Other things being equal, gear oils which will have the lowest internal friction will be the most desirable from a power efficiency standpoint.

In connection with friction, consideration might also be given to the oiliness characteristics of gear lubricants. The following definition was adopted  by the SAE  Crankcase  oil  Oiliness  Committee  in  1937: Oiliness  is  a  term  signifying differences in friction greater  than can be  accounted  for  on a  basis   of  viscosity  when  comparing different lubricants  under identical  test conditions.

This quality is  desirable in  boundary lubrication  and  is  no doubt  due to adsorbed layers  on  the metal  contributed by  polar compounds  such as fatty acids, esters  of  fatty acids, metallic soaps, some  organic  compounds containing  chlorine, nitrogen, phosphorus, or sulfur, etc. Unfortunately there is  no  standard method   for  measurement of  oiliness.

Conditions under which gear lubricants operate

With meshing gears, both rolling and sliding motions are present. However, the  two  types  of  contacts  vary  both  with  the  type  of  gears  and  the  speed  of  operation. The  sliding  component  is  of  greatest  importance  in  the  case  of  hypoid  or  worm  gears. The descriptions of the state of lubrication, which  immediately  follow, are concerned, to a large  extent, with the types of  gears  used  in  most  industrial  applications, transmissions  of  automotive  vehicles, etc. Some  of  the  thinking  and  speculation  can  also  be  applied  to  hypoid  and  worm  gear  operation.

Many  gear  lubricants  operate  under  very severe  conditions  and  yet  long  and trouble  free  lives  are  obtained  from  most  gear  sets. In spite  of  the  fact  that  the  action  of  such  lubricants  is not  completely explained, enough  information  is  available  to  permit  recommendations  of  gear  oils  which  will  perform  satisfactorily  provided  the  gear  design  and  installation  is  not  at  fault.

Borsoff^10  has  presented  the  mechanism  of  gear  lubrication  in  a  simple  descriptive  from  which  is essentially  as  follows. As  gears  rotate  in  the presence  of  a  lubricant, a hydrodynamic  wedge from, which tends  to separate  the  teeth  as  they  mesh  with a  thick  fluid  film  when  the  load  is  low. As  the load  increases, the  pressure  in  the  contact  zone  also  increases, causing  the  separating  Lubricating  film  to  decrease  in  thickness. Finally,  the  load  becomes great  enough  that  the fluid  film  fails  to  prevent  contact  of  the  high  spots  of  the  mating  surfaces, and  wear  results.

The  investigator^10   found  that  the  nature  of  the  were was dependent  upon  speed  and  that  gear  lubrication  can  be  divided,  with respect  to  speed, into  three  zones. First  is  the slow  speed zone, which  in  the  gears used  in  the investigation, extends  up  to  about 1000 rpm. Next, the medium  speed  zone  for  the  same gears  extends  approximately  from  1000  to  8000  rpm. Finally,  the  high  speed  zone  extends from  8000 to 30,000  rpm  in  the  case  of  the  investigation.

Three  regions  were  recognized  with  respect  to  type  of  wear  and  working  surface  conditions, namely: a region  of  thick  film  lubrication  where  there  was  absence  of  wear; a region  of  abrasive  wear; and a  region  of  scoring.

In  the slow  speed zone, the load  carrying  ability  of  a  given  lubricant  increases  with  decreasing  speed. Thus, using  an  oil  of  9.92 cs  at 100 degree F, no scoring  took  place  at  speeds  below  665 rpm. However, wear does  occur  at slow  speeds as loads  increase, and  under  these  conditions  it  was concluded  that  such  wear  was  due  to  abrasion.

Gears  operating  in  the medium  speed  zone  with light  loads  were  in the thick film region, and  hence wear was not  detectable. As  loads increased  in this  speed zone, heat  generated  by shear  caused  a  decrease  in  the viscosity  of  the  lubricant  followed  by a rupture  of  the fluid film with consequent  metal  contact  and scoring. It was found that this type of wear was not gradual  but increased  by jumps  at the beginning of each  load  period, after  which no additional  wear  was  observed  until  another load  increase .Under  these  conditions, the lowest viscosity  oil  permitted  the  greatest  wear, which also  started  at low  loads. The high  speed  zone was ‘characterized  by an increase in load  carrying  capacity  with  an increase  in  speed.’’ This was attributed   to two factors, one the relaxation phenomenon and the other the ‘squeeze’’ effect. Relaxation in this sense indicates that the liquid  lubricant  responds  as  an elastic  solid  when  subjected  to  high  deformation  rates. Since  a  definite  length  of  time  is  necessary  to squeeze all of  the  lubricant  out of  the contact  zone  between  gear teeth ,  at  high  speeds  the  contact  time  may be  too  short  to  eject  all  of  the oil. According  to Borsoff^10 “ high  speed  gear  operation  at  all  loads  below  the  score  load   is  in  the  thick  film  region.”The  above  mechanism  of  gear  lubrication  was  concerned  with  lubricants  consisting  of  unreactive  mineral   oils.   It  was  found  that the  higher  the  viscosity  of  the  lubricants, the greater  the  load carrying  capacities  and  wear  protecting  properties. However, other requirements often dictate the use of low viscosity oils. Since operation  of  gears  using  unreactive  mineral  oils  at  loads  above  their  first  score   load  may  lead  to  trouble, the  use  of  extreme   pressure  (EP)  lubricants  is  found  necessary   with  high  loads.

Borsoff^10  cautions  that  ,  when  using  EP   lubricants  at  loads  above    the  score  load  of  the base  oil, abrasive  wear  may  sometimes  be  present.  Likewise,  abrasive   wear  may  occur  with  heavy  loads  in  the  slow  speed  zone .  Such  action  and  the  degree  will  depend  upon   the  particular  EP  additive  as  well  as  the   concentration. In  order to  appreciate   more  thoroughly   the  problem  of   gear  lubrication,  the ideas  of   other  investigators  should  have  consideration. According to  MacConochie  and  Newman^37  contact  pressures  of  7000  to 10,000  kg/sq  cm  in  the  case  of  gears  compare  with 10 kg/sq  cm  for  journal  bearings . Also , instead  of  an  oil  film  thickness  of  about  0.001  cm , as  is present  in journal  bearings, the  lubricant  film  formed  between  meshing  gear  teeth  is  of  the  order  of  magnitude  of  the  surface  roughness  of  the  two  contacting  bodies  and  not  much  thicker  than  the  size  of  foreign  particles  in  a  highly  purified  oil  passing  through  the gap. The  function  of  the  oil  is  performed  in an  extremely  short  time  since, according  to  Smith^46,   the  residence  time  of  the  lubricant  in  the  contact  zone  may  be  as short  as 10^-6  seconds.

The  first  authors^37  visualize   the   conditions  under  which  gear  oil   films  operate  as  follows: ‘Sometimes  there  is  a  substantial   lubricant  film  between  the  surfaces, and  at  others  a  foreign  particle  wedges  its  way  through   between  two  surface  roughnesses , at other  times  two  or  more  asperities  come  into   physical  contact. Depending  upon  the  relative  waviness  of  the  surfaces  and  the  difference  in  particle  size,  contact  may  occur  only  at  one  point  along  the  contact  line  while  lubrication  is  hydrodynamic  in  other  regions. Another  factor  affecting  thickness  readings  is  the size  of  the  contact  zone  since  the  larger  the  contact  area  the  greater  the  chance  for  a  particle  of  large size  to be  in  the  gap.  To complicate the picture   further,  the  film formed   is  constantly  subjected  to  dynamic  load  on  the  gear  train, vibration  of   the  gear  teeth , and  shafts. Localized ‘conflagrations’ resulting in chemical reactions may occur at heavy loads.’’

Oil film thickness was measured by these investigators^37 using a continuous electrical arc to bridge the gap between gear teeth. A tracing  of the results indicates  that the lubricant  film  is  at  a  maximum  at  the  pitch  line  and at a minimum at  the roots  or tips  of  the gears,  see Fig.2.1.

Much remains to be explained regarding the lubrication of gear sets. For example, in a discussion  of  the report  of  “Instantaneous  Coefficients  of  Gear Tooth  Friction’’ Benedict  and  Kelley^4  state: “We   do  not know  from  our  results  the  true  state  of  lubrication, whether  it  be hydrodynamic, elasto-hydrodynamic, or partial hydrodynamic. In  our  analysis, we have chosen  partial  hydrodynamic  lubrication  as  a  simple  model  which   helped  visualize  the results. In order to determine precisely  the  state  of  lubrication  it  would appear necessary  to  determine  by  some  more  direct  observation  not  only  the thickness  of  the oil  film but  also  its  continuity’’.

Also  gears  are  used  under  such variable  conditions  that  a  blanket  statement  cannot  be  made  as  to  the  type  of  lubrication  which  prevails. This was recognized by the above investigators^4 as   follows :Lightly loaded high  speed gears  might  reach  full  fluid  film  lubrication, and  heavily  loaded  low  speed  gears show  signs  of  being  in the  boundary  region. These are  extremes, however, and in the results  discussed  here  and  in  gears  normally  used, mixed  lubrication  occurs.

In  view of  the fact  that most  gear  sets  operate  trouble  free, the  extreme conditions  indicated  by  MacConochie  and Newman^37 may  not  for average operations. Thus, Crook^19 who  used electrical  resistance  as  a  means  for measuring  the  thickness  of  oil  films  between  metal  dises, concludes  that  hydrodynamic  films  of  one  micron  in  thickness  result after  a  break  in  period. This author states: Furthermore, the existence of a hydrodynamic film is quite consistent with observations with gears themselves. Often signs of the original  machining  marks  can be seen even  after twenty  years  service  and such low  rates  of  wear  imply  hydrodynamic  rather  than boundary  lubrication.

Perhaps  a  new lubrication region  should  be  recognized  which Talley  and  Givens^47  have  defined  as ‘metadynamic’ and which  falls  between  the  hydrodynamic  and  boundary  regions. These  authors, in dealing  with  such  a  lubricating  film, have  measured  one  aspect  of  oiliness  and  derived equations describing  the  same. While  the  investigation in  question  was  concerned  with  journal  bearings, one  interested  in  the  theory  of  gear  lubrication  should  be  aware  of  this  thought.

Some of the complexities   of  boundary  lubrication  as  suggested  by Larsen  and  Perry^35a  may occur  during  gear  operation, particularly  if  high   temperatures  are  reached. Any  of  the  following  reactions  might  have  an  influence  on   gear  lubrication: oxidation  of  the  metal  surface; oxidation  of  the  lubricants  to  from  fatty  acids; chemical  or  physical  adsorption  of  polar  compounds, such  as  fatty  acids, on the  metal  surfaces; formation  of  multilayer  films  by  the  adsorption  of  the  above  fatty  acids, by  salts resulting  from reaction  of  the  acids  with  metal  oxides, or  by esters  present; oxidation  or polymerization  of  oils  or  unsaturated  constituents  to  from  resinous  films; orientation  of  these latter  films  due  to  pressure  and  shearing  stress; or  a  breakdown  of  any of  the  films  just  described.

While  the conditions  under  which  gear  lubricants  operate   seen  to  be quite  severe, it  should  be  realized  that  average  operating  conditions  are  less  serious. Producers  of  both gear sets and  gear  lubricants  provide enough  tolerance  in  their  products  so  that  difficulties  are  the  exception  rather  than  the rule. However, the possibility   of  conditions  such  as  those   cited  behooves  the  operator  of  gears to  provide  as  reasonable  service  conditions  as  possible  so as  to prevent  undue  wear  or  even  failure.

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.