Insane Hydraulics

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Does Bigger Displacement Always Mean Bigger Torque For a Hydraulic Motor?

I get this "myth" a lot from our clients. It concerns orbital hydraulic motors, and if I were to sum it up with a single phrase, it would be "bigger = stronger", however, since the hydraulic equipment user base uses the word "stronger" to universally describe both the notions of power and torque, I like to subdivide this myth into:

This myth usually crops up when an orbital-motor-based hydraulic system requires an upgrade, and it is assumed that replacing the driving orbital motor with one from the same series but with a bigger displacement will provide the required "force increase".

There's solid logic behind this assumption. First of all - using the same frame size eliminates the hassle of having to adapt a new flange and shaft, which is extremely convenient, and second - since the torque of a hydraulic motor is, essentially, its displacement times pressure, surely a simple step-up in displacement will make my winch stronger, will it not? Hell, I'll even upgrade the damned pump if I need to!

Well, my upgrade-thirsty friend, I'll need to know the exact model of your current orbital hydraulic motor to tell you if a displacement step-up will do what you want it to do. Why? Because for some orbital motors, the maximum torque is not defined by displacement!


No, not blasphemy! A mechanical limitation rather, because orbital motors have a "torque fuse" built into them by design - the cardan shaft:

Orbtal motor cutaway view showing the cardan shaft

While the torque-generating rotor can grow in size, the torque-transmitting cross section and the splines of the cardan shaft can't, which is why for many orbital motor series (note - "many" does not mean "all"), the maximum allowable torque distribution across the available displacements for a given frame-size looks like this:

Danfoss OMPX and OMRX max continuous torque distribution across available displacements

This torque chart is for the Danfoss OMPX and OMRX motors, and you can see how the maximum allowable torque flattens at 400 Nm starting at 200 cm³. So, if your winch employs an OMR100, bumping it up to 160 will provide a healthy 38% (maximum) torque upgrade (275Nm => 380 Nm), however, increasing the displacement past 200cm³ (while keeping the OMRX frame) will not provide a safe increase in torque. The key word here is safe, by the way. Obviously, if you are one of those intrepid fellows whose motto is "intermittent is the new continuous" - nobody can stop you from finding out how conservative were the OEM's safety margins when they published the torque values.

This "max torque flattening" towards the end of the displacement range is very common for orbital motors similar in size and construction to Danfoss's OMP (gerotor) and OMR (geroller) motors. Here's a table with max. continuous torque values for OMPX and OMRX motors along with the values for their Char-Lynn counterparts (namely the H-series and the S-series). You can see that the H-series flats out at the similar 400-ish Nm (while offering the ridiculous displacement option of 739 cm³ - and I can't really imagine an application for this motor), the S-series lets the maximum allowable torque climb along with the displacement till the end of the displacement lineup (possibly because of a slightly stronger construction of the cardan shaft), but you can still see that at the end of the displacement range a 25% displacement increase (298 cm³ => 372 cm³) allows for a mere 7% increase in maximum allowable torque (433 Nm => 465 Nm):

cm³ OMPX OMRX H series S series
25 40
32 50
36 64
40 52
46 84
50 110 100
59 103 114
74 134
75 147
80 170 215
97 176 182
100 210 275
120 219 233
125 270 330
144 266
146 268
159 275
160 335 380
166 304
185 339
187 331
200 400 400
225 369
231 319
250 400 400
293 351
298 433
315 400 400
370 407
372 462
375 400
400 400 390
739 389

Now - for the second part of the myth - the power - i.e. the kilowatts a hydraulic motor can "dish out". Once again, the power distribution across a displacement lineup looks like this:

Danfoss OMPX and OMRX max continuous power distribution across available displacements

This, of course, is related to the rpm limitations of larger motors, but it is still very counterintuitive to accept that a 100 cm³ motor can transmit twice as much power as its 400 cm³ sibling (once again - I am talking about orbital motors and I am comparing motors that belong to the same frame size).

This, essentially, means that very often you can't skip upgrading the frame size of an orbital motor when you need a "system boost".

Oh yes, almost forgot, there's one more thing one should bear in mind when assessing the torque capability of an orbital motor - the output shaft - because the output shaft can also be a torque limiting factor, especially if it is one of them "fancy" shafts, like the 1'' cross-hole, which is only good for 200 Nm (never used it and never will). I guess one could even say that orbital motors are too strong for their size.

Be it as it may - in such situations, the motor technical catalog is (as always) your best pall that will tell you what you can and can't do.