Helicopter Gear Lubricating Problems

Since the gearing in question is high speed, reference to a previous section, which deals with problems in lubricating high speed gears, may indicate similar problems. Also an investigation by peck et al.^39 is an aid in this regard. Under high speed conditions, good results were obtained by lubricating with a “jet hitting the center of the gear tooth on the outgoing side of the tooth mesh”. Also baffles were used to prevent swirling of the oil with resulting heating. Likewise, baffles were necessary to direct oil to the suction lines “without being picked up by high velocity airflow's and whirled around”. The characteristics of suitable oil for lubrication of highly loaded helicopter transmission, turbo-prop reduction gears, variable pitch propeller gearing etc., are similar to those required in aircraft turbine oils. Therefore, if lubricating problems occur, they will be similar in both instances.

Power Take off Tractor Lubrication

Farm  tractors  from  different  sources  employ  various  types  of power  take off  devices. Therefore, a common recommendation cannot be given for lubrication of such equipment. In  order to  regulate  the  speed  of the  PTO  independently  of the  speed  of the  tractor, clutches  or a  different  set  of gears  may be   used. With  gears  alone, the grade  and type  of gear oil used  for the tractor  transmission may  be  encountered  and in this  case  ATF  may  be  used. The best  procedure is not  depend  upon  the  recommendations  of the  supplier  of  the  machine.

Garden Tractors and Gear Lubrication

The gears in most garden tractors can be lubricated with engine oil. This should preferably be at least an SAE 30 in weight. Any classification of automotive oil available will be satisfactory. At the end of the season the gear case should be drained and refilled with fresh oil.

  

Automobile Assembly Lines and Gear Lubrication

The conveying  machinery  of  automobile  assembly  plants is  driven by gear  reduction  units  which  are  powered  by  electric  motors. Floor level conveyors may have variable speed transmission units and spur gear trains in order to obtain the very slow rate of travel desired. The  speed  reducers first  mentioned  are enclosed and  lubricated  with  grades of oil  conforming  to AGMA  recommendations for the average  speed   of  the gear  train. Spur gears driving the floor conveyors may be too large to house economically and will be lubricated by intermittent spray of SAE 90 gear oil. Gear lubrication in additional facilities of automobile factories or of auxiliary plants supplying parts to assembly lines will be treated elsewhere.

Alcohol and Brewing Industries and Gear Lubrication

The raw material for either alcohol or beer is to a large extent, grain with lesser amounts of molasses or sugar. The use of speed reduction gears starts with the handling of the grain. This  may be  by  conveyors  of  various  types  which, if not  chain  driven, will  be driven  by gear  reducers. Often the grain will be screened before going into storage bins. The screens, which may be of the reel or shaker type, can be driven by gear reducers. A suitable  gear  oil  for  the  above  operations is one  of about  500 viscosity SUS at 100 degree F, equivalent  to  AGMA lubricant No.3.

In a  brewery  might be  found barley sieves  driven  with  open  gears, grading  screens  with similar drives, melting drums  likewise  driven  with  open  helical  gears, malt  grinders with  gear  driven  rolls, spent  grain presses  with  open  gearing, hop  extruding  machines  having  enclosed  gears,  beer  centrifuges  with  enclosed gears, etc. The  open  gearing  in the  above  applications  should be  lubricated  with  a  residual  oil  having a  viscosity of  about  2000 SUS at 210 degree F. This  should  be  applied sparingly  and if there is the  least possibility  of contamination  of  the  material  being  processed, a  shield  might  be installed  to  prevent  this. Enclosed gears can be  lubricated with well  refined oils of 200 to  500 SUS at 100  degree F. Centrifuges  should  use  the lower  viscosity  grade or  an  equivalent  to  AGMA  lubricant  No. 1 or 2.

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.

Bath or Splash Application of Gear Lubricants

Enclosed gear sets are often lubricated by bath or splash application. For this purpose, the gear oil is much more fluid than that used for open gears and hence not only lends itself to such methods of application but also acts as a coolant. Such systems are cleaner and more economical than many methods of application since only a tight housing is required.

These methods of lubrication are automatic since the oil feed starts with the start of the gears and ceases when they are shut down. These systems are efficient and reliable if the oil is kept at the correct level. Further, a minimum of attention is required.

Experience and location of the gear sets dictate the size of the gear case. Where the cases are small compared   to the size of the gears, as is the case in most vehicles, the amount of oil is also restricted with a consequent restriction in cooling effect. In addition, close fitting gear casings can cause excessive oil drag which in turn will increase heating.

Satisfactory lubrication is obtained by bath or splash methods of gearing up to peripheral speeds of 2500 to 3000 feet per minute and occasionally at higher speeds where the operating time is comparatively short.  The amount of oil carried to the point of mesh will depend upon the gear size, the speed, and the viscosity of the   lubricants and to some extent upon the construction of the gear case. As speeds increase, a point will be reached where centrifugal force will overcome surface tension and viscosity and most of the oil will be thrown off. This may finally reach a point where little lubricants will adhere to gear teeth at the point of mesh. In case an excessive amount of oil does reach the engaging teeth operating at high speeds, the lubricants will be displaced at considerable velocity and cause noise. According to Merritt^37 this condition is likely to occur with double helical gears with continuous teeth operating with the apex trailing.

Such methods of application may include the following:

(1)    By bath where the fluid is placed in a sump or bath in the bottom of the enclosed gear case. One of the gears dips into the bath and as this gear rotates it transfers the lubricant to the contacting teeth. Any excess is thrown against the housing and is guided, by means of troughs, either into the bearings or back to the sump. It is important that the gear case should not be too full and that the oil level be determined, if possible, when the gears are idle. Excess lubricant leads to foaming and causes excessive drag and heating. It is suggested that in industrial gearing the oil level should be such that the bottom gear dips into the oil about three times the depth of the tooth spaces. Automotive vehicles generally provide a filler opening and the gear oil should be up to the bottom of this opening; or, if a filler neck is used, the oil should be at the top of this filler neck. It is important that vehicle gear cases be filler when gears are stationary and also that the car or truck is on a level floor.

(2)    By splash, using either one of the gears or an auxiliary flinger which dips into the bath of gear oil and throws the lubricant toward the top of the gear case so that it drops back into both gearing and bearings. Merritt^37 states: “ All dip lubricated gears produce  different oil levels  at different points of running, and the running levels indicated by an oil level  gauge  may be either  higher or lower than the standing level.”

This again shows the necessity of still filling gear cases.

(3)    By idler gear. In some instances, particularly on slow moving gear sets, an idler gear dips into a sump and transfers oil to the contacting gears.

While mention is made of bearing lubrication, in bath lubrication the gear oil may or may not also serve the bearings. It is important to recognize this in determining what viscosity or grade of oil to use since bearings normally require lighter oils than do gears. A compromise as to viscosity may therefore be necessary if the lubricant serves more than one function.

                         The lower teeth of exposed gears are sometimes encased so as to permit bath lubrication. Such application is only practical when speeds are relatively slow, otherwise considerable oil would be thrown out of the gears

Hand application of Gear Lubricants

Hand  application of gear lubricants, while often used on slow speed  exposed gearing, should  be discouraged because  there is  always  a certain  amount of hazard to  such a procedure, the amount  of lubricant used is indeterminate, and  the operation is wasteful of both time and lubricant. However, primarily because of the lower initial cost, open gears have considerable use. Also, both location of the gears and immediate economy, dictate hand application of lubricants to such gears. Consequently, some of the methods of hand application follow:

(1)    By paddle, when the lubricant is a heavy residual type or consists of a solid or stiff grease. Usually more of the lubricant than is necessary is applied and consequently the excess either   drops off or is thrown off into the surrounding area. This is not only wasteful but also contributes to poor housekeeping.

(2)    By hand in case the residual type is heavy enough. Such a product can be formed into a ball by hand and then dropped into the gear mesh where in time it is distributed.

(3)    By bags, made of plastic, most often polyethylene, which is filled with the gear lubricant. A filled bag is dropped between the gear teeth where eventually it is chewed to pieces by the gears and the lubricant is released. This makes a somewhat cleaner operation than the previous methods and the hands are not soiled while applying the lubricant. If a large opening is provided in a gear case, this method of application can sometimes be used for enclosed gears.

(4)    By pouring from a can, bucket, or any spouted container. Such applications are made at intervals and in case of gear oil is very heavy, it may require heating to reduce the consistency before applying. The lubricant is then poured in a fine stream into the teeth as the gears slowly revolve toward each  other. If the oil is applied relatively frequently in small amounts rather than at longer intervals in large amounts, there will be less chance of its being thrown off.

(5)    By hand spray at intervals, most often from “ AerosoI” containers. Such a package, which ordinarily holds up to a pound of a diluted gear compound, makes for ease of application and cleanliness because an excess of the lubricant is not applied. The gear compound is diluted with a non-inflammable solvent which evaporates rapidly after release from the container and leaves a viscous coating on the gear teeth. While the diluents present may be toxic, the amount released at one application is normally so small that it dissipates without harmful effects.

Gear lubricants as structural materials

Since gear sets would not function without a proper lubricant, it has been suggested that gear oils be given the status of structural materials. As early as 1942 Almen^1 stated that a gear lubricant in addition to being: “A lubricant in the usual accepted  sense of a liquid film separating two rubbing surfaces,” should also be considered as “a structural material in the sense that it is an important factor in determining the size of gears.”

More recently, Blok^7a devoted an entire paper to “Gear Lubricants as Constructural Elements”. This author suggests that the designers of gear sets keep in mind that:

“The lubricant is to be conceived as a constructional material, and thus its constructional properties, such as viscosity and antiscuffing properties, well deserve to be accounted for even in an early design state”.

Consequently, Blok^7a plotted curves connecting power transmitted with speed, showing the barriers which must be raised either by improvement in materials or by the use of special lubricants if load capacity of gears is to be increased

Introduction

Almost  all  modern  machinery  makes  some  use  of  gears  as  a  medium  for  transmitting  power. Such use  may  be  for  the  purpose  of  either  changing  the  speed  of  a  driven  member  or  changing  the  direction  of  movement. In  either  case  the  gears  act as a lever  so  that , in  most  cases, the  force  transmitted  is  multiplied  and  in  so  doing  speed  will  be  reduced. When  variations  in  the  torque  or  speed  transmitted  are  desired, a  series  of  gear  ratios  can  be  provided  in  the  same  mechanism.

Most  gear  sets  are  lubricated  with  fluid  products  or  gear  oils, and  consequently, such  compounds  will  receive  major  consideration . Since  plastic  or  semi-fluid  products, such  as  lubricating  greases, as  well  as  solids  are  sometimes  used  as  gear  lubricants, they  will  also  receive  mention.

It  is  also  the  purpose  to  describe  fluids  employed  in  certain  fluid  drives, particularly  automatic  transmissions  in  automotive  vehicles. Here  the  oils  in  question  serve  a  number  of  functions, and  it  is  necessary  that  a  single  fluid  satisfy  not  only  lubrication  of  different  mechanisms, but  also  act  as  a  torque  converter  and  hydraulic  agent.

This  is  simply  an  extension  of  the  functions  which  gear  oils  fill  in  many  case. Thus, gear lubricants   also take care of bearing lubrication in many   applications. It will  be  presumed  that  most  readers  are familiar  with  the  mechanisms  to  be  lubricated, such  as  automatic  transmissions, hypoid  gears, worm  gears, etc. Therefore, little description of mechanical details will be attempted.

The  primary  interest  will  be  in  lubricants  for  metal  gears, although  mention  will  be  made  of  the  requirements  for  gears  made  from  other  materials. An  attempt  will  be  made  to  thoroughly   explain  how  lubricants  for  gears  function, what  they  contain, how  they   are  compounded, and  the  limitations  of  their  use. In  all  of  this  it  should  be  kept   in  mind  that  to  be  most  effective   any  lubricant  should  be  applied  in  the  right  amount  at  the  right  place  and  the  right  time.

Fundamentally  the   types   of  motion  in  operating  gear  sets  are  those  occurring  in  other  machine  elements , namely, rolling  and  sliding  motion. The two types of movement   may occur simultaneously as in meshing   hypoid gears. This  is  pointed  out  so  that  it  will  be  realized  that, even  though  the  configuration  of  gears  differ   from  bearings, the  lubricating  problems  are similar  for  both  mechanisms. Another  point  to  keep  in  mind  is  that  while  the  title  of  the  mentions  both  gear  and  transmission  lubrication, the latter  mechanism  is  most  often  one  or   more  gear  sets  serving  a specific  purpose. Therefore, except  in  the  case  of  fluid  drives, the  problem  is  one  of  gear  lubrication. Finally , some of  the  possibilities  of  future  trends  and  developments  in  gear  lubricants  and  torque  fluids  will  be  mentioned.