Technical trends

THE PM TECHNICAL sessions

that were organised as part of

the 2004 SAE World Congress

at Cobo Hall, Detroit, in

March covered a number of advanced

technology automotive engineering topics.

As usual, some of the leading edge devel-

opments in automotive PM applications

were given prominence in the programme.

However, the relatively low-tech automo-

tive oil pump also received some rare

attention.

Automotive oil pump components have

been manufactured by PM for several

decades. Both the inner and outer rotors

for the various types of pumps have tradi-

tionally been made from ferrous-based

powders by conventional pressing and sin-

tering, followed by sizing and final

machining.

Now, following the current industry

trend, PM parts fabricators are increas-

ingly being asked to supply complete

pumps and even to get into the design of

pump systems with improved efficiency

for higher pressure applications. Efforts in

these directions were the subject of two

papers in the PM sessions.

Several approaches to improving

pump efficiency were discussed by

Bernhard Terfloth, of GKN Sinter

Metals, based in Radevormwald,

Germany, and a team of six other

authors from that company.

For gear-type oil pumps, one approach

that these authors looked at was the gear

tooth design. They compared three tooth

profiles: involute and cycloid profiles for

seven-tooth gears, and a cycloid gear with

nine teeth (Figure 1). Testing under simu-

lated engine conditions showed that the

involute tooth profile had the lowest

mechanical and volumetric efficiencies

and the highest noise level, while the nine-

tooth cycloid profile gear gave both the

highest efficiency and lowest noise. It was

concluded that a change to the nine-tooth

cycloid profile gear design would improve

oil pump performance and enable smaller

pumps to be designed with the same

pumping capacity.

Further improvement of oil pump

gear performance was shown to be

achievable by gear-rolling of the sintered

tooth surfaces instead of axial sizing in a

die. As well as reducing surface rough-

ness by an order of magnitude, the

gear-rolling process densified the tooth

surfaces and improved dimensional

accuracy and hardness resulting in

improved pump efficiency without extra

production cost.

Another approach discussed by

Terfloth and colleagues was to look at

alternative pump designs. In this respect

the benefits of the GKN-patented plane-

tary rotor pump design (Figure 2) were

listed. This pump design can be used

with pressures up to 160 bar and was

said to compete with vane- and crescent-

type pumps. In addition to its higher

efficiency, it was much quieter in opera-

tion. Based on the geometry of a conven-

tional gerotor pump, the planetary rotor

Improving automotive oil pumps - by designTeams looking at refining established PM product lines have shown that reassessment ofdesign and manufacturing methods can bring improvements to even the most familiar items...

30 MPR May 2004 0026-0657/04 ©2004 Elsevier Ltd. All rights reserved.

Figure 1. Pump gears with different profiles and numbers of teeth (After Terfloth et al.)

Shape Related Efficiency Aspects (1)

• optimising tooth profile of external gears

• no great affect on the PM process or tooling cost

• investigate tooth geometry on pump efficiency

• different tooth profiles were compared

• standard testing procedure developed to mimic engine operation

condition (26-80°C, 1-6bar, 500-5000rpm)

Involute (7) Cycloid (7) Cycloid (9)

pump has small planet gears that provide

self sealing, reducing oil leakage and

providing high volumetric efficiency. The

superior performance of the planetary

rotor pump makes it suitable for several

auto applications, including automated-

shift gear boxes, double-clutch transmis-

sions, continuously-variable transmis-

sions, slip-coupling for locking differen-

tials in four-wheel drive, and other appli-

cations demanding high efficiency in a

small package.

The final aspect of pump design dis-

cussed by these authors was the material

of construction. One of the concerns

here is that the use of cast aluminium

pump housings, favoured for weight sav-

ing, would increase internal leakage

problems due to differences in thermal

expansion between the steel rotors or

gears and the aluminium housing.

Increased leakage from the axial clear-

ance, reducing efficiency as the pump

warms up, can be compensated by the

use of bushings. Another solution to this

problem would be to use a suitable PM

aluminium alloy for the rotor, with a

coefficient of thermal expansion com-

patible with the cast aluminium housing.

GKN Sinter Metals has developed such

an alloy, containing 14% silicon, 2.5%

copper, and 0.5% magnesium. This alloy

has a coefficient of thermal expansion

closely similar to that of the aluminium-

silicon cast alloys used for pump hous-

ings, and in addition shows improved

wear resistance. Examples of the use of

this new PM alloy in several sizes of

pump gerotors were given.

Tooth profile design for gerotor-type

oil pumps was the subject of a presenta-

tion by Shinichi Fujiwara of Mitsubishi

Materials Corp. He reported on a project

in which the tooth profile of sintered

rotors for high efficiency oil pumps was

modified to achieve several design

improvements. Without getting into the

nitty-gritty of the new profile design, he

showed how it improved flow rate by

7 per cent through smoothing out the

fluctuations in flow velocity with

rotation angle. The new tooth profile

also reduces friction loss and the reduced

working clearance (one-third of the

standard design) results in lower

operating noise.

Joe Capus

metal-powder.net May 2004 MPR 31

Technical trends

FULL texts of the written papers from

the PM technical sessions have been

published by SAE International in

Special Publication SP-1847 "Powder

Metallurgy-Power Transfer and Net

Shape Solutions" obtainable from SAE

International, 400 Commonwealth

Drive, Warrendale, PA 15096-0001,

USA. Tel: +1 724 776 4841; Fax: +1

724 776 5760. Website: www.sae.org

Further information

Figure 2. Traditional gerotor (LHS) compared with GKN planetary rotor. (After Terfloth et al.)

Concept Related Efficiency Aspects (2)

• main geometry based on a g-rotor pump

• designed to create a self-sealing macro/micro gear

interface, where in conventional g-rotors pumps most

leakage occurs

• thus volumetric efficiency is mainly determined by

axial clearance

Leakage due to clearance

Self-sealing micro gear