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My understanding of swash plates led me to believe that the pistons in a swash plate pump rotate around with the driving shaft along with the chambers in which they aspirate (e.g. M&S Hydraulic Youtube video on swash plate plumps). As the axial position of the pistons change, the piston chambers draw and expel fluid.

However, I just watched a training video from "Our Virtual Academy" (which costs money to view unfortunately) where the pistons do not rotate with the shaft; instead, the swash plate rotates causing the pistons to move axially. The angle of the swash plate is varied by forcing pressurized fluid into the control chamber (here is an example).

I can't imagine why one would use this latter form of swash plate pump. What are the reasons for the different approaches? (What is the correct terminology to distinguish the two so I can carry on my own research)

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  • $\begingroup$ Instead of youtube, why not check out the manufacturers of those pumps - they usually have good information $\endgroup$
    – Solar Mike
    Jun 10 '19 at 6:04
  • $\begingroup$ There are some differences between pumping and motoring, such as the need to self-start a hydraulic motor, which means you need a bunch of pistons radially arranged. A pump can be built with just one cylinder. Sealing a pressurized and noxious gas is harder than sealing hydraulic oil that just bleeds into a sump and is piped back to a tank. The energy wasted in compression makes this a bad idea in gasses. So go with a fixed cylinder that can be sealed and rotate the swashplate. plate. $\endgroup$
    – Phil Sweet
    Jun 11 '19 at 9:32
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There exist three types of axial plunger pumps.

1- Rotating swash plate axiale plunger piston pumps:

Usually for low-power systems, as the temperature rises above 120° C, it can degrade the fluid properties and cause some damages to the sensitive components.

the advantage of the first type is it simplicity and efficiency, driving a light swash plate requires less power than the heavy block of barrel and pistons

2- Fixed swash plate axiale plunger piston pumps:

In this type, the cylinder barrel rotates with the shaft, of course, designing this type needs more consideration and have more complexities than the previous type, but the clearance between the cylinder barrel and the housing block, allows leakage of the fresh fluid so it serves a lubricant of the rotating body and cooling of the system.

3- Bent axis axial plunger piston pump:

The cylinder barrel makes an angle with the rotating shaft, by changing the angle, you can acquire a variable displacement pump.

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  • $\begingroup$ Can you tell me which pump is which from my original question? $\endgroup$
    – Jordan McBain
    Jun 10 '19 at 14:00
  • $\begingroup$ And the second of mine? $\endgroup$
    – Jordan McBain
    Jun 10 '19 at 14:02
  • $\begingroup$ @JordanMcBain The first mechanism, you described is number 2 and 3 in my answer, and the second mechanisme according to you is number 1 in my answer. $\endgroup$
    – Sam Farjamirad
    Jun 10 '19 at 14:07
  • $\begingroup$ If your first description has the problems it has, why wouldn't one always use the third system you described? $\endgroup$
    – Jordan McBain
    Jun 10 '19 at 14:08
  • $\begingroup$ Actually, I've never seen any practical pumps of the first type, is it possible to build it, and study it academically. The third and the second mechanisms are more popular anyway. $\endgroup$
    – Sam Farjamirad
    Jun 10 '19 at 14:10
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Following up on Sam's categories of pumps -

  1. There are industry examples of your first category - rotating swash plate + stationary barrel pumps. Here is one animated example of a pump that I've used personally - https://www.youtube.com/watch?v=sP3gc4b8Z5k. The relevant part of the video starts at 0:48. Pardon the marketing material. The pump's barrel must remain stationary because each piston chamber is connected directly to an outlet check valve. As they show at 1:30 in the video, the concept of a stationary barrel allows for separate flows from a specialized cover plate, which isolates 1 or more piston chambers from the others. A similar "isolation" concept is used by Bosch-Rexroth, Hydac, and HAWE for multi-chambered radial-piston pumps.

  2. This is the most common variety of axial piston pump. The barrel / housing (valve plate) clearance does indeed allow leakage for pump cooling as you describe. Note that this is only 1 of 4 typical leakage sources in a rotating-barrel pump... enter image description here ($Q_{lp}$) piston / bore clearances, ($Q_{ls}$) slipper / swashplate clearances, ($Q_{lv}$) barrel / valve plate clearance, and (not pictured) swash plate control system return oil.

  3. Good description on the bent-axis category.

Overall your post categorizes axial-piston pumps correctly into the 3 general groups. However some of the descriptions are only true in specific situations.

  • For pump category 1, you mention 120C as an upper operational limit. 120C is an extreme upper limiting point for petroleum-based hydraulic oil used in any type of axial piston pump. However there is no single number for temperature limit. It completely depends on the working fluid used with any specific pump design. Each fluid has a unique relationship between temperature and relevant fluid properties - viscosity, density, compressibility. Also, each pump may have interfaces with unique clearances optimized for a particular fluid. Pump failure often results from viscosity falling below some functional limit (1.0-10 cSt) due to temperature rise. Also pump failure can be caused by temperature alone in the case of elastomer seal degradation. Lastly note that bent-axis type pumps (category 3) may survive longest at elevated temperatures because they are designed without piston slippers for axial load support. Less sliding components that can fail..

  • Category 1 is not inherently simpler or more efficient than the other categories. It's true that the barrel / valveplate sliding interface is eliminated in Category 1. However multiple new valves are introduced for periodic sealing of each piston chamber. Additional bearings are needed to support the side-load on the rotating swashplate. Also far more expense is involved to achieve variable-displacement operation. Lastly the weight of the rotating group has negligible influence on power consumption. Component weight only affects the accel/decel time needed to change speeds. Steady-state power consumption is a function of viscous fluid shear on surfaces, along with any power diverted for displacement control purposes.

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  • $\begingroup$ I have the barrel (from a Rexroth paxial pump) in front of me now - used as a pen stand and given to me after I was there for a project. It is designed with 7 pistons and definitely rotates with seal faces and the ports designed to match chambers at one end. So not stationary as you mention in 1. $\endgroup$
    – Solar Mike
    Jan 7 '20 at 17:54
  • $\begingroup$ Some axial piston pumps operate like the Rexroth you have on your desk. The barrel rotates with the shaft. Others have a stationary barrel. The accepted answer described those varieties as category 1 and category 2. So I kept the same terminology. $\endgroup$
    – Jeremy Beale
    Jan 7 '20 at 18:16

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