- Email: [email protected]
PM gears get to grips with hydraulic pumps
PM gears get to grips with hydraulic pumps Since its foundation in 1903, Burgess-Norton Mfg Co (B-N) has been a leading manufacturer of precision metal components. B-N% involvement with powder metallurgy (PM) dates back to 7957, with the company continuing to play a central role in the development of the technique for gear fabrication. In this review article, Glen A. Moore and Todd Km Olson of 6-N explore current applications for PM gears in hydraulic pumps, discuss various manufacturing techniques and touch upon the future of PM gearing.
A
s manufacturers in industries as diverse as automotive and material handling equipment continue to seek means of reducing cost without compromizing design or performance integrity, the future of powder metal (PM) components appears very bright. Indeed, the Metal Powder Industries Federation (MPIF) now estimates North American sales alone at more US$2 billion, and worldwide shipments in excess of one million US tonnes. Spanning market boundaries in particular is the growing utilization of PM gears in hydraulic pump applications. From low pressure engine lubrication pumps to higher pressure industrial applications, PM gears satisfy a triangle of key design criteria.
FIGURE
1: A selection
of PM
gears for a variety
of hydraulic
pump
applications.
30 MPR November
1997
0026-0657/97/US$17.00 All rights reserved.
Types of PM gears Despite today’s popular manufacturing mantra of ‘cost reduction’, engineers at all levels have not lost sight of the fact that components must meet performance criteria before other design considerations may enter the specifying decision fray. As increases in dimensional accuracy of PM have outpaced those of conventional gear hobbing methods, so has the performance of PM gears. Use of PM gears can be divided into three basic segments covering low, medium and high pressure applications. Each segment has its own product niches with its own set of performance and manufacturing requirements. Low pressure applications: This segment covers applications operating at less than 690 kN.m-’ (100 psi) and has long been suited for PM gear usage. The typical seven tooth, six pitch gear used in engine lubricant pumps are commonly found in internal combustion engines. Gears of this variety are frequently pressed from MPIF FC-0208, a common copper steel, to a 6.4 g.cmm3minimum density without requiring heat treatment. Standard tolerance requirements of 0.05 mm (0.002”) for involute profile, 0.08 mm (0.003”) for the outside diameter (OD), 0.05 mm (0.002”) for length, and 0.03 mm (0.001”) for the inner diameter (ID) are well suited to relatively inexpensive calibrating processes during manufacture. Gears of this type, therefore, may be produced in highly automated environments. Because of the relatively generous density requirements for low pressure applications, the gear may be easily mated with a pump shaft through a shrink operation. Medium pressure applications: These gears are used in pressure ratings ranging from 690 to 10 350 kN.m-’ (100 to 1500 psi) and are typically of the design shown in Figure 2. Common uses for gears in this classification include lighter hydraulic applications such as lawn tractors or material handling equipment. Similar to their low pressure brethren, gears operating within these pressures ranges fit well within the realm of PM gear capabilities. Some differences do exist, however, in the manufacturing processes. For the part shown in Figure 2, MPIF nickel Copyright
cl 1997, Elsevier
Science
Ltd.
steel FN 0208 has heen used and pressed to a minimum density of 7.0 g.cm “. Heat treatment allows this gear to satisfy the minimum surface hardness of RC 35 and minimum partic’lr hardness of RC 55. Although this gear can be manufactured without a costly secondary coining operation. several integral manufacturing elements merit menlion. For the parts illustrated in Figure 2, the involute profile tolerame has been held closer than that of the low pressure gear and a keyway has been added to secure the gear after it has been drivrn on to the pump shaft. This mating operation also necessitates t,he 15 ID chamfer on one end of the gear. Additionally, t.he ID, OD, length and geometric charact erist its require secondary machining operations which must be performed after heat treatment. Lastly, the much tighter lead error tolerance of 0.04 mm (0.00 15” ) requires extremely careful monitoring during all manufacturing processes. Despite t,hcl more tightly toleranced characteristics and result,ant secondary operations, competent PM houses specializing in gears are more than capable of cc,onomically producing gears of this calibre. High pressure applications: This segment includes I’M gears used in ratings up t.o and in excess of 20 700 kN.m ’ (3000 psi).
It represents
the most sophisticated PM pump gears produced through conventional PM manufacturing techniques. Figure 3 illustrates a typical high pressure pump gear produced from conventional MPIF low alloy steel to a minimum density of 7.4 g.cm ‘I. Heat treatment renders an apparent minimum hardness of HRC 45 and a minimum particle hardness of Kc: V5.5. Because of the high pressure environment, in which this gear is designed to
FlGURE
2: Typical
PM gears
for use in medium
Ljespite differences in their manufacturing processes, all PM gears share a common competitive advantage over traditionally hobbed gears price. The average cost, savings generated versus traditional gears per piece part is 25.50’1;), according to estimates undertak:en by Tom Stockwell, N’s sales manager of PM parts.
volume production economies of scale
The economics of PM gears
B-
applications.
Much of this savings relates directly to the efficient, near net shape, manner in which PM parts are manufactured, which considerably reduces scrap material. For example, a bevel gear weighing one pound after manufacture requires one pound of powder metal. That same gear produced by traditional hobbing may have been cut from bar stock weighing 1.5 pounds. Because the basic material costs on a per pound basis are similar, engineers designing with PM have already generated a 33’% cost savings. Rates of hourly production further enhance the value oEered by PM gears. In terms of pieces per hour, a typical PM gear can be produced at the rate of 500 pieces per hour. Compared to the 30 or 40 gears which can be bobbed hourly, one can quickly appreciate the economies of scale which the PM process offers. It should be noted that because of the inherent tooling costs, roughly US%15 000 per set up, PM gears are best suited to high
operate, the increasingly stringent OD, ID, length, parallelism, lead and involute tolerances require secondary operations. Tighter lead error tolerance and the 0.04 mm (0.0015” ) involute profile often necessitate comlbacting and coining operat ions. wpical applications for gears of this variety are pumps used in large off road, construction and agricultural equipment.
pressure
applications where can be maximized. MPR November
1997
31
HGURE
3: PM gears
for high
pressure
applications.
Bearing in mind the material savings and increased hourly production, one can readily surmise how quickly these tooling costs pay for themselves.
The future of PM gears in hydraulic pumps Just as engineers have met past challenges in designing stronger PM gears, and as
32 MPR November 1997
PM manufacturers have worked to overcome initial skepticism of their product, new hurdles must be overcome to make PM gears the universal product of choice for all hydraulic pumps. Well suited to applications up to 20 700 kN.m-’ (3000 psi), PM gear technology is on the threshold of producing gears capable of withstanding 27 600-34 500 kN.m-’ (4000. 5000 psi). Several new manufacturing techniques hold great promise in creating this next generation of PM gears. Gear tooth finishing, improved powder compaction techniques and new materials are likely to be driving factors in this evolution, Because of the successes of PM gears, several manufacturers are currently involved, through their MPIF representative, with the American Gear Manufacturer’s Association to compile two documents regarding PM gear standards. The first, “AGMA Standard 6008-AX?’ is nearing completion. The second, “AGMA Information Sheet 9xX-Axx: Calculated Load Capacity of PM Gears” is underway. These documents will provide heretofore unavailable aid and guidance to pump designers n in the use of PM gears.
A
s manufacturers in industries as diverse as automotive and material handling equipment continue to seek means of reducing cost without compromizing design or performance integrity, the future of powder metal (PM) components appears very bright. Indeed, the Metal Powder Industries Federation (MPIF) now estimates North American sales alone at more US$2 billion, and worldwide shipments in excess of one million US tonnes. Spanning market boundaries in particular is the growing utilization of PM gears in hydraulic pump applications. From low pressure engine lubrication pumps to higher pressure industrial applications, PM gears satisfy a triangle of key design criteria.
FIGURE
1: A selection
of PM
gears for a variety
of hydraulic
pump
applications.
30 MPR November
1997
0026-0657/97/US$17.00 All rights reserved.
Types of PM gears Despite today’s popular manufacturing mantra of ‘cost reduction’, engineers at all levels have not lost sight of the fact that components must meet performance criteria before other design considerations may enter the specifying decision fray. As increases in dimensional accuracy of PM have outpaced those of conventional gear hobbing methods, so has the performance of PM gears. Use of PM gears can be divided into three basic segments covering low, medium and high pressure applications. Each segment has its own product niches with its own set of performance and manufacturing requirements. Low pressure applications: This segment covers applications operating at less than 690 kN.m-’ (100 psi) and has long been suited for PM gear usage. The typical seven tooth, six pitch gear used in engine lubricant pumps are commonly found in internal combustion engines. Gears of this variety are frequently pressed from MPIF FC-0208, a common copper steel, to a 6.4 g.cmm3minimum density without requiring heat treatment. Standard tolerance requirements of 0.05 mm (0.002”) for involute profile, 0.08 mm (0.003”) for the outside diameter (OD), 0.05 mm (0.002”) for length, and 0.03 mm (0.001”) for the inner diameter (ID) are well suited to relatively inexpensive calibrating processes during manufacture. Gears of this type, therefore, may be produced in highly automated environments. Because of the relatively generous density requirements for low pressure applications, the gear may be easily mated with a pump shaft through a shrink operation. Medium pressure applications: These gears are used in pressure ratings ranging from 690 to 10 350 kN.m-’ (100 to 1500 psi) and are typically of the design shown in Figure 2. Common uses for gears in this classification include lighter hydraulic applications such as lawn tractors or material handling equipment. Similar to their low pressure brethren, gears operating within these pressures ranges fit well within the realm of PM gear capabilities. Some differences do exist, however, in the manufacturing processes. For the part shown in Figure 2, MPIF nickel Copyright
cl 1997, Elsevier
Science
Ltd.
steel FN 0208 has heen used and pressed to a minimum density of 7.0 g.cm “. Heat treatment allows this gear to satisfy the minimum surface hardness of RC 35 and minimum partic’lr hardness of RC 55. Although this gear can be manufactured without a costly secondary coining operation. several integral manufacturing elements merit menlion. For the parts illustrated in Figure 2, the involute profile tolerame has been held closer than that of the low pressure gear and a keyway has been added to secure the gear after it has been drivrn on to the pump shaft. This mating operation also necessitates t,he 15 ID chamfer on one end of the gear. Additionally, t.he ID, OD, length and geometric charact erist its require secondary machining operations which must be performed after heat treatment. Lastly, the much tighter lead error tolerance of 0.04 mm (0.00 15” ) requires extremely careful monitoring during all manufacturing processes. Despite t,hcl more tightly toleranced characteristics and result,ant secondary operations, competent PM houses specializing in gears are more than capable of cc,onomically producing gears of this calibre. High pressure applications: This segment includes I’M gears used in ratings up t.o and in excess of 20 700 kN.m ’ (3000 psi).
It represents
the most sophisticated PM pump gears produced through conventional PM manufacturing techniques. Figure 3 illustrates a typical high pressure pump gear produced from conventional MPIF low alloy steel to a minimum density of 7.4 g.cm ‘I. Heat treatment renders an apparent minimum hardness of HRC 45 and a minimum particle hardness of Kc: V5.5. Because of the high pressure environment, in which this gear is designed to
FlGURE
2: Typical
PM gears
for use in medium
Ljespite differences in their manufacturing processes, all PM gears share a common competitive advantage over traditionally hobbed gears price. The average cost, savings generated versus traditional gears per piece part is 25.50’1;), according to estimates undertak:en by Tom Stockwell, N’s sales manager of PM parts.
volume production economies of scale
The economics of PM gears
B-
applications.
Much of this savings relates directly to the efficient, near net shape, manner in which PM parts are manufactured, which considerably reduces scrap material. For example, a bevel gear weighing one pound after manufacture requires one pound of powder metal. That same gear produced by traditional hobbing may have been cut from bar stock weighing 1.5 pounds. Because the basic material costs on a per pound basis are similar, engineers designing with PM have already generated a 33’% cost savings. Rates of hourly production further enhance the value oEered by PM gears. In terms of pieces per hour, a typical PM gear can be produced at the rate of 500 pieces per hour. Compared to the 30 or 40 gears which can be bobbed hourly, one can quickly appreciate the economies of scale which the PM process offers. It should be noted that because of the inherent tooling costs, roughly US%15 000 per set up, PM gears are best suited to high
operate, the increasingly stringent OD, ID, length, parallelism, lead and involute tolerances require secondary operations. Tighter lead error tolerance and the 0.04 mm (0.0015” ) involute profile often necessitate comlbacting and coining operat ions. wpical applications for gears of this variety are pumps used in large off road, construction and agricultural equipment.
pressure
applications where can be maximized. MPR November
1997
31
HGURE
3: PM gears
for high
pressure
applications.
Bearing in mind the material savings and increased hourly production, one can readily surmise how quickly these tooling costs pay for themselves.
The future of PM gears in hydraulic pumps Just as engineers have met past challenges in designing stronger PM gears, and as
32 MPR November 1997
PM manufacturers have worked to overcome initial skepticism of their product, new hurdles must be overcome to make PM gears the universal product of choice for all hydraulic pumps. Well suited to applications up to 20 700 kN.m-’ (3000 psi), PM gear technology is on the threshold of producing gears capable of withstanding 27 600-34 500 kN.m-’ (4000. 5000 psi). Several new manufacturing techniques hold great promise in creating this next generation of PM gears. Gear tooth finishing, improved powder compaction techniques and new materials are likely to be driving factors in this evolution, Because of the successes of PM gears, several manufacturers are currently involved, through their MPIF representative, with the American Gear Manufacturer’s Association to compile two documents regarding PM gear standards. The first, “AGMA Standard 6008-AX?’ is nearing completion. The second, “AGMA Information Sheet 9xX-Axx: Calculated Load Capacity of PM Gears” is underway. These documents will provide heretofore unavailable aid and guidance to pump designers n in the use of PM gears.