Between gears and pistons

Alternative designs, such as internal gear pumps, can serve applications that demand more than what external gear pumps can deliver without the expense of axial-piston pumps.

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By Frank Pirri,
Flo Draulic Group, Toronto

Figure 1. Eckerle design of internal gear pump shares elements common to standard designs, but contains a channel in the crescent and additional features that improve volumetric efficiency.

Figure 2. A cross-section of a new external gear pump is illustrated on the left, and an external gear pump with typical wear is illustrated on the right. Wear between the housing and gear teeth creates internal leakage that decreases volumetric efficiency. Leakage increases over time and at higher pressures.

When you need high performance and long life, the first choice is normally an axial-piston pump. When efficiency and performance demands are less crucial, but economy is paramount, external gear pumps generally get the nod. So what if you want higher efficiency and longer life than a gear pump can provide, but an axial-piston pump may be overkill or not quiet enough?

The Eckerle design of internal gear pumps serves as a viable alternative to common external gear pumps and piston pumps in many applications. Figure 1 shows the Eckerle internal gear pump, design, with blue representing low-pressure fluid and red representing high pressure. As with standard internal gear pumps, a spur gear meshes with an internal ring gear to pump fluid from the inlet port to the outlet. In operation, the spur gear drives the ring gear, and both rotate in the same direction. A crescent element seals the pressure chamber and acts as a bearing between the spur gear and ring gear teeth.

Even the standard design of internal gear pumps runs quieter than common external gear pumps. This is because internal gear pumps have a film of oil between the rotating ring gear and pump housing. This film of oil tends to suppress noise and vibration from the meshing gear teeth and also keeps the ring gear separated from the external housing, so less noise is transmitted to the surrounding environment. Furthermore, because the spur and ring gears have more teeth meshing than two spur gears do, pressure pulses are less severe than with external gear pumps.

However, standard internal gear pumps generally are limited to only moderate pressure and must be driven at relatively high speed. This is because internal leakage between the gear teeth and the crescent is relatively high and only gets worse as pressure increases. Likewise, internal leakage also occurs between the ends of the gears and end plates of the pump.

Compensating for pressure and wear
The Eckerle internal gear pump design shares the same basic components as standard internal gear pumps. However, the Eckerle design contains a longitudinal channel within the crescent. Pressurized fluid inside the channel tends to push the two halves of the crescent apart. The force from this pressure improves the seal between the lower half of the crescent with the spur gear teeth, and the upper half of the crescent with the ring gear teeth. A pin near the center of the crescent acts as a positioning dowel and blocks fluid from escaping from the high-pressure chamber to the low-pressure chamber.

A similar type of compensation is designed into the end plates to improve volumetric efficiency between the ends of the spur and ring gears and the inner surface of the end plates. Both of these improvements permit the Eckerle design of internal gear pumps to accommodate intermittent pressure to 400 bar (5800 psig) and continuous pressure to 300 bar (4785 psig). Furthermore, because the crescent and end-plate designs reduce internal leakage, Eckerle design pumps don't have to be driven at high speed. The standard speed range is 400 to 4000 rpm.

Higher operating pressure has minimal effect on the volumetric efficiency of Eckerle internal gear pumps. This is because of the pressure balancing features inherent to the design that automatically decrease clearances with increasing pressure.

These same features contribute to longer pump life by automatically compensating for wear. Efficiency of these pumps initially increases slightly, (running-in of the parts to each other) and then remains constant. On the other hand, external gear pumps, Figure 2, offer no means of compensating for wear. So an external gear pump's volumetric efficiency decreases the longer it operates under load.

So the next time you need to specify a pump for an application, maybe you shouldn't automatically choose between an external gear pump or piston pump. Many alternative designs exist — including, but not limited to, internal gear pumps. One of these may be just what you need to reduce noise and increase efficiency and life of the equipment you design.

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