Continuing the discussion from last week, the past fifty years saw a continued evolution of the gear pump, which has resulted in higher efficiency, higher pressure and more versatility than was ever previously available. Aluminum alloys are now the most common construction material for the gear pump housing, and hardened steel is used for the spur gears. Pumps can now be modelled in 3D with CAD software to ensure a quality design even before the first pump is prototyped, and with CNC machining employing fine tolerances, finished parts are optimized before they reach the assembly station. However, this is the nature of most manufacturing these days, so this is not unique to gear pumps or even hydraulics in general.
The previous limitations in gear pump construction prevented efficient high-pressure operation. The physical limitations of the pump, such as the strength of the metals and seals, dictate absolute pressure maximum, but it’s the design and tolerances that dictate usable maximum pressure. Older pump designs with too much space between the spur gears and the pump’s interior walls allow fluid to bypass to the suction side rather than be forced out the pressure port. It is that bypass which is the primary factor in pump efficiency, and even the highest quality, brand-new gear pumps have difficulty cracking 85% efficiency, so you can imagine the difficulty faced by your grandfather at the mill. Increasing pressure past a certain point just wastes more energy as heat.
Advances in gear pump design are more than just those related to improved efficiency. Although reliability has always been high for gear pumps, especially considering the level of fluid contamination they can withstand with no effect on performance, improvements have been made regardless. Older pump designs had their spur gear stub-shaft mount directly in the pump housing, which created more friction than needed. Current models, in the very least, have bushings to support the shafts, improving reliability. I’d like to say the bushings also improve serviceability, but even the most expensive gear pumps are not worth the time it takes to repair them, and are considered throwaways. Some high-end pumps use needle bearings to support the shafts of the drive and idler gear, which improves mechanical efficiency.
Some new gear pumps use a bushing sleeve in the shape of a number 8 on either end of the spur gear set. This sleeve acts as a bushing, but also channels leakage to the suction side of the pump, which negates the requirement of a case drain. This special sleeve allows the gears to switch sides along with the mounting flange, while maintaining the position of the drain on the suction side. This technique reverses the rotation of the pump, while pumps without this option must be manufactured in both clockwise and counter clockwise versions.
So far in this article, most of the differentiation between yesterday and today has been evolutionary. Gear pumps have not shown to experience huge jumps in technology or concept, and they are remarkablely similar to ones Grampa was using back in the day. However, even lowly gear pumps are not immune to advancements in design and technology.
One of the downsides of the gear pump is the noise. It’s not just that they tend to be loud, but they often have an annoying frequency that makes you want to jump off a bridge. Bosch Rexroth developed its Series S or “Silence” gear pump to combat pressure wave noise of the spur gears. Typical gear pumps rely on the meshing on two spurs the full width of the gear pitch, but the spur of a Series S pump acts upon only the width between two opposing flanks, which smooths out the annoying noise of the pressure waves.
If that wasn’t enough, Bosch Rexroth took it a step further and developed their Silence Plus version of this pump. By using helical gears rather than straight-cut spur gears, the pressure waves are nearly eliminated because no single large spike is created as the gears rotate. The pressure wave is smoother and less parabolic, reducing both noise and vibration. This type of pump is the pinnacle of hydraulic gear pump technology, and with advanced materials and construction, is very lightweight and efficient as well. This is most definitely not your grandfather’s gear pump.