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Application of P/M Gears in Hydraulic Pumps

By Glen Moore, Tom Prucher, Mark Green, John Engquist, Pat Kenkel and Steve Haye

Burgess-Norton Manufacturing Company

Abstract

The powder metal process has been used for the fabrication of various types of gears for many years. These include spur, bevel, helical, and in many cases, combinations of the above (cluster gears). Each of these involute gear types possesses some special processing techniques which allow them to be divided into different special categories. The purpose of this paper is to discuss precision powder metal external spur gears, which have been found to operate satisfactorily in a variety of hydraulic pumps. There has been an increasing trend towards the use of the powder metallurgy process in fabrication of external spur gears for hydraulic pumps, which operate in the pressure ranges from 40-3,000 psi. This trend is mainly due to the better economics, as well as increased dimensional accuracy of powder metallurgy over conventional gear hobbing methods.

Introduction

Involute curves are used in the construction of gear teeth in order to promote smooth conjugate action as the teeth slide and mesh with each other. In the early years of powder metallurgy, involute curves were actually generated by unraveling a piece of string from a circle called a base circle diameter. This form was usually magnified 10X and by various means transferred to an electrode, which was subsequently used to ram EDM the gear form into a powder metallurgy compacting die. Similar methods were used to manufacture punches, which would precisely fit into this die, allowing compaction of the gears to occur. While at the same time, this method was used to construct powder metal tooling, similar more expensive wrought steel gears were hobbed on special gear hobbing machines from a billet of barstock. It was debatable as to which method, powder metallurgy or hobbing of gear teeth, was the more accurate. One fact which was known was with the conventional hobbing method, one first machined the bore and then, locating on the bore, hobbed the gear teeth, while in the conventional P/M method, one located on the gear teeth and precisely located the bore.

With the invention of numerically controlled wire EDM machines, the accuracy of powder metal gear teeth was greatly enhanced. The involute curve derived from the same base circle could now be conveniently programmed and very accurately wire EDM machined in a powder metal compacting die. Typical tolerances held utilizing this method are .0002 " all over, with a microfinish of 8 microinches. This methodology, combined with major advances in powder metal materials, as well as modern gear finishing techniques, is largely responsible for the increased demand for powder metal gears used in hydraulic applications today.

Typical External Gear Hydraulic Pump Construction

In most cases a hydraulic pump consists of two end plates and a center body section which houses the gears. As a side note, in many cases these components are also made out of powder metal. Inside this housing is typically one precision powder metal gear, known as the drive, which is fastened to a wrought steel shaft, and another powder metal gear known as the idler, in many cases identical to the first, which spins freely on the other shaft. Both gears are necessary for the pumping action.

The fluid actually being pumped is taken in at the suction port and carried into the two compressing action caused by tooth meshing induces pumping pressure.

It should be obvious that the accuracy of the gear teeth, which includes the lead error, or the parallelism of the teeth to the bore, and the O.D. tolerances, must be kept to a minimum or pressure leakage will occur. The same concept applies to the overall length of the gear as it relates to the opening provided by the end plates of the pumps. Actually, it will be shown later that the end clearances are typically more important than the radial clearances.

In order to have a completely closed hydraulic circuit in the pump, the teeth themselves must seal against pressure along the line of the surface contact. This is another reason the involute tooth form is chosen for hydraulic pump gear design.

Hydraulic Pump Gear Classirications

The authors have chosen to divide this discussion into three separate categories:
Low, medium, and high pressure pump gears.

Low Pressure Pump Gear. Gears fitting into this category (less than 100 psi) represent the largest quantity of gears produced by the powder metallurgy process. This type of gear is commonly used to pump engine lubrication oil. The material, WEF FC-0208, is a copper steel pressed to a 6.4 minimum density, non-heat treated. The .002 involute profile tolerance is fairly generous for the powder metallurgy process. It should also be noted that the .003 O.D. tolerance combined with the .002 length and the .001 I.D. tolerance lend themselves to less expensive calibrating processes in the manufacture of these gears. These gears are typically manufactured in highly automated environments.

Medium Pressure Pump Gear. Typical characteristics of a medium pressure pump gear (less than 1,500 psi). Note that the authors have selected a common MPEF nickel steel, FN 0208, at a 7.0 minimum density, heat treated to an RC 35 minimum. The particle hardness will exceed RC 55 minimum. It is quite important to note that this 7.0 minimum precision gear can be manufactured without an expensive secondary coining operation. The involute profile tolerance is held closer than on the low pressure gear, and a keyway has been added in order to drive the gear on the shaft instead of the shrink fit that was found on the low pressure gear. The close I.D., O.D., length, and geometric characteristics require secondary machining operations after heat treat. Also worth mentioning is the lead error tolerance of .0015, which requires extremely careful monitoring of all processes in the manufacture of this gear. There is a 150 I.D. chamfer on one end for assembling the gear on the mating shaft, and a 450 O.D. chamfer on one end, which provides clearance between the gear and the mating radius in the bottom of the pump body.

High Pressure Pump Gear. The high pressure pump gear, which is usually rated up to 3,000 psi and in some special cases higher, is the most sophisticated of all the P/M pump gears produced using conventional P/M processing. The authors have chosen to produce this gear from a conventional MPEF molybdenum steel with a 7.3 minimum density, heat treated to an apparent hardness of HRC 45 minimum. T'he particle hardness of this particular material would be RC 55 minimum. Because of the .0015 involute profile and lead error tolerance, this type of gear frequently requires a sizing operation in addition to the compacting and coining operations. The tight tolerances on O.D. and I.D. length and parallelism require special secondary operations.

Materials

The blended iron-nickel alloys have been the workhorses of the P/M industry and helped make the introduction of the P/M gear a success. Both FN-0205 and FN-02098 have been used successfully. The hardenabihty of the iron-nickel alloys, however, is low when compared to the pre-alloy powders. In some cases, pre-alloy powders have been required to achieve a more consistent heat treat response. FL-4605 was an early substitute for the blended iron-nickel alloys. The introduction of the straight molybdenum alloy FL 4405 replaced both FN 0205 and FLA605. The advantages of the FL 4405 are lower cost and better compressibility than the FL 4605. With FL 4405, higher density is more practical, and the steel is more hardenable than with FN 0205. Eventually, the value of the admixed nickel was rediscovered. Both FLN 4205 and FLN 4405 have become popular today, usually with 2% nickel powder admixed as in the original BN 0205.

The medium and high pressure pump gears are heat treated to achieve high strength and high hardness. For the different pump applications, higher pressure output requires stronger teeth. The increase in strength is achieved with an increase in density rather than changing the alloy. For low pressure engine oil pump applications, 6.4 g/cm3 minimum density is satisfactory. With an increase in pressure output, density up to 7.0 g/cm3 is satisfactory. For high pressure pump applications, a coined gear at 7.3 g/cm3 minimum density is satisfactory.

Additional Information

As is typical in many manufacturing processes, tolerances held by conventional P/M processes in some cases are still not good enough for certain applications. Other auxiliary operations have been found to be acceptable.

Involute Tooth Form Improvement. In those instances where an involute profile tolerance of less than .0015 and a lead error tolerance of less than .0015 are required, subsequent finishing operations can be performed. This modern technology, when applied to P/M gears, is extremely cost-effective compared to conventional cut gear techniques.

Root Fillet Radius. In many instances the volumetric efficiency of a P/M gear is superior to that of a cut gear because of the root fillet radius. An undercut is commonly found on a cut wrought steel gear, which is the result of the hob and hobbing machine geometry. With the powder metal gear, it is easy to construct a full fillet radius, which starts approximately ten thousandths below the base circle diameter, all of which is easily machined by wire EDM into the powder metal tooling.

Roll Chart. In most cases elemental gear checking; i.e., O.D., root diameter, measurement over wires and concentricity, are suitable for most P/M gear processing inspection. In cases where tolerances are much tighter, it is accepted practice to roll the work gear against a precision master gear and note the deviation from nominal. A skilled operator can read the tooth-to-tooth, total composite and index errors on a typical roll chart. This technique is usually accompanied by a center distance specification, with an upper and lower tolerance limit.

In order for precision pump gears to run quietly, positive involute profile traces must be avoided. Due to the growth and shrinkage of some P/M materials from die size, this gear characteristic must be checked frequently by electronically traced involute profile checking equipment. It is not unusual to keep the involute negative above and below the pitch line in order to reduce noise and enhance wear.

Gear Shaft Assemblies. As mentioned earlier, P/M gears are especially attractive economically compared to cutting a solid gear shaft component out of bar stock. There are many ways to provide a robust P/M gear-wrought steel shaft assembly. This usually involves an interference fit whereby the I.D. of the gear and the O.D. of the shaft are held to .0003 tolerance each. This process sometimes involves a simple interference fit and at other times includes a keyway.

Summary

It was the objective of this article to present an overview of typical P/M gears used in hydraulic pump applications. A brief discussion of typical pumps, involutes, materials, and gear testing methods was presented while discussing characteristics of low, medium and high-pressure P/M pump gears. P/M researchers continue to work on advanced material systems as well as processing techniques, which hopefully will allow P/M gears to withstand loads required of pumps in excess of 4,000-5,000 psi.

References:
1. "Modem Methods of Gear Manufacturing," National Broach & Machine, Division of Lear Sigler Inc., 1972.
2. Andreotti, E. R. and S.W. McGee. "Performance of P/M Gears in Hydraulic Pumps," Report, Burgess-Norton Mfg. Co., 1968.

This article was reprinted with permission from Advances in Powder Metallurgy & Particulate Materials, Metal Powder Industries Federation, 105 College Road East, Princeton, New Jersey, USA, 1996.

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