The following episode happened just a couple of weeks ago and is worth describing because it addresses an interesting application scenario for an orbital hydraulic motor, and also the sometimes not so obvious differences between the "original" units and cheap knock-offs.
Over the last decade, low priced hydraulic components, mostly made in China, have flooded the European market. If you take a look at any business directory, you will find hundreds of Chinese suppliers selling copies of "real" hydraulic brands, from spare parts to complete pump/motor units. And, of course, the most common type of a low-speed high torque hydraulic motor is no exception.
The main advantage of Chinese-made motors is, of course, their price, which being a fraction of the cost of the original attracts resellers like sugar attracts ants. And although some Chinese manufacturers do produce hydraulic motors of acceptable quality, I would still strongly advise a sheer amount of exhaustive sample testing before you even consider ordering any Chinese-made hydraulics. Even after that, you should be morally prepared for "surprises".
Despite all that, life teaches us that it is newer wise to row against the current, and rejecting Chinese hydraulics means giving market to competition, so many, if not all, European hydraulic companies stock low-priced OMR/OMP knock-offs to satisfy the demand. Most of the time such copies perform just like the original motors at half of the price, but not ALL of the time...
This time a client was claiming warranty replacement of two OMR125s (copies, not original) which according to him "weren't turning". It was an unexpected claim, as, first of all, these motors had been bench tested before shipment, and the same client had bought similar motors before, which performed flawlessly on identical equipment.
The motors were tested again, showing normal efficiency and no problems whatsoever. Despite that, the motors were disassembled "just in case", and no wear or visible damage was detected. Then they were reassembled, re-tested, and prepared to be shipped to the client with the advice to look for a problem elsewhere in the circuit.
When the motors were just about to be handed over to the transport agent, the region sales-man casually mentioned a new fact - these two motors were operating in parallel, driven by a simple small power-pack. One word caught my attention - small... When I wondered HOW SMALL it was, it turned out the output flow of the pump was around 1.5 liters per minute, which meant one thing - cancel the delivery, let's re-test the motors under the newly discovered low flow conditions! Before that new information (go figure!), the motors had been tested with a nominal flow-rate.
Although such motors are "high torque low speed", it is normal for them to present certain speed ripple at extremely low speeds., which is why catalogs reference minimum speed and also such thing as a minimum starting torque, which is LOWER than the torque at nominal speed with the SAME delta P. The rotor position, and the commutator valve (which in these motors is the machined shaft, often referred to as the "spool") position contribute to this phenomenon.
When the motors were tested under the low flow, one of them appeared to be stalling in certain positions of the shaft with all the flow (about 1 lpm) by-passing through the motor with a mere 30 bar delta P. Which meant that with the flow that low, any braking torque applied to the motor shaft high enough to raise the system pressure to 30 bar would be stalling it! The system pressure wouldn't be able to rise because of the cross-port leakage inside the commutator valve. The phenomenon was not noticeable at higher flows. Then I tested an original Danfoss OMR-125, and it clearly showed a much less pronounced speed ripple (although still present), and no stalling points. By the way, Sauer-Danfoss references the minimum speed of 9 rpm for these motors, which equals roughly the flow of 1.2 liters per minute.
Let's look into this low-speed ripple phenomenon. A common OMR has a 7-tooth outer ring and a 6-tooth rotor that make seven "work chambers". Normally two or three of them are pressurized, two or three are connected to the return, and one or two are in the "transitional" state. In that particular motor, probably due to lower machining tolerance, the commutator valve was slightly "under-lapping", thus creating the leak between the ports in the "transition zone". When the supplied flow rate became comparable to the leak rate, the leak itself became the effective pressure limiting factor. Of course, this wasn't an issue at nominal flow rates.
The problem was solved by testing several equal motors and choosing the two with the least noticeable speed fluctuation at the given low flow. It also must be mentioned that the price of the two motors was still lower than the price of a single Danfoss.
This example is perfect to show that lower price comes at the cost of quality and that a copy is never as good as the original.
It also shows that choosing motors for very low speeds or stall conditions has its peculiarities. Low speeds bring along the jerky movement, and the fact that the starting torque can be significantly lower than the nominal torque should be taken into account when you choose the motor's size and brand.