The following episode happened just a couple of weeks ago, and is worth
describing because it addresses several interesting questions concerning
the so-widely-spread low speed high torque gerotor/geroller type
hydraulic motors, as well as small, and sometimes not so obvious
differences between original motors and cheap copies.
Presently, low priced and mostly Chinese made
hydraulics has literally flooded the European market. If you take a look at any
business directory, you will find hundreds of Chinese suppliers selling
copies of all existing "real" hydraulic equipment brands, from
spare parts to complete pump/motor units. And, of course, the most
common and cheapest type of hydraulic motor, namely gerotor/geroller
type is no exception.
The main advantage of Chinese-made motors is their
price, of course, which being a fraction of cost of an original motor
attracts European resellers like sugar attracts ants. And although some
(not all, oh no...) Chinese manufacturers do produce motors of
acceptable quality, I would still strongly advise a sheer amount of exhaustive sample
testing before ordering any Chinese made hydraulics. Even after that
you should be morally prepared for unexpected surprises.
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
copies along with original ones, to satisfy the "poorer-folks" demand.
Most of the times such copies perform just like the original motors at
half of the price, but not ALL of the times...
This time a client was claiming warranty replacement of
two OMR125 geroller type motors, 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 prior to 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, retested, 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 brought up in a conversation
a 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 retest the motors under the newly discovered low flow
conditions! Before that new information (go figure!), the motors
had been tested with nominal oil flows.
Although such motors are high torque low
speed, it is normal for them to present certain speed rip or unstable
jerky movement at extremely
low speeds. That is why in some catalogues you will find minimum
speed reference, and also such thing as minimum starting torque, which
is LOWER than maximum torque at nominal speed with the SAME lines delta
P. The rotor position, and the commutator valve (which in these motors
is the machined shaft) position contribute to this phenomenon.
When the motors were tested with the given low flow,
one of them appeared to be stalling at certain positions of the shaft
with all the flow (1 lpm) passing through the motor creating only 30
bar delta P. Which meant that with the flow that low, creating the amount
of shaft torque sufficient to raise the system pressure to 30 bars was
enough to stall the motor. The system pressure wouldn't rise more
because of the A to B leak inside the commutator valve. The phenomenon
was not noticeable at higher flows. Then I tested an original Danfoss
OMR-125 motor, and it clearly showed a much less noticeable speed rip
(although present), and no stalling points. By the way,
Sauer-Danfoss state the minimum speed of 9 rpms for these motors, which
would equal roughly 1.2 lpm flow.
Let's look into this low speed rip phenomenon.
A common gerotor motor has a 7 tooth outer ring and a 6 tooth rotor,
thus creating seven working chambers. Normally three are pressurized,
three are connected to tank, and one is in "transitional" state,
passing from T to P. In that particular motor, probably due to
machining error, (or lack of quality control, or lack of quality
equipment, or disrespect to tolerances, or lack of working conditions,
or all of the above and a lot more...) the commutator valve was
slightly underlapping, thus creating the small leak between the lines
in the "transition" chamber groove. 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 two with the least noticeable speed
fluctuation at the given low flow. It also must be said that the price
of the two motors was lower than the price of one original.
This example is perfect to show that lower
price comes at the cost of quality (both machining and material), 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 jerky movement, which sometimes can be unacceptable, and for some
motors, like gerotor/geroller type motors, there is a minimum starting
torque, which is lower than the maximum torque - something that
should be taken into account when sizing such motors to create torque
under stall/start conditions.