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Hydraulic motor case flushing

Hydraulic motor case flushing

Hydraulic motor case temperature

At the latest my project I used hydraulic motor Rexroth A2FLM710 (710cc). The motor works at 1400 rpm and provides 590 HP to the consumer. Foe safe motor work I always try to keep the case temperature below 80*C. The easiest way to do this is a flushing flow adjustment.

In addition to the flow, you need to keep eyes on a case pressure and try balancing to prevent overpressure in the motor case (check in the motor manufacturer’s catalog the max available case pressure to make longer life of motor shaft seals).

The values I came are 21 GPM at 30 psi case pressure and in the worst-case scenario (max motor load, warmed hydraulic oil) case max temperature was around 80*C

Hydraulic motor case flushing

Hydraulic motor case flushing flow and case pressure

There no prescriptions or recomendations for valve or orifice size in motor catalogs for flushing flow, so the selection of flushing valves is a challenge.

Of course, you can find orifices (with different diameters) provided by the manufacturer with the motor in the motor’s catalog. But the flow and result case temperature will be different from application to application and the selection of correct orifice is an engineering responsibility without any help or advice from motor manufacturer.

Moreover, the manufacturer can’t provide all range of orifice diameters so the selection in the catalog is usually limited. And as you can see, sometimes values of flushing flow can be really huge and the only experience helps me to select the right flushing valve size at the beginning of the project.

I still believe, manufacturers can provide some diagrams/charts with correlation power-> flushing flow for approximate/preliminary estimation of the flushing valve size. Because I do not think everyone has a chance to make long tests during production and play with valves sizes…

What do you think?

Charge pump displacement

Close loop circuit

There is a generic rule for close loop systems, to select the charge pump size: pump displacement should be at least 10% of the combined displacement of the pump and motor. To understand where this value comes from let’s figure-out all factors in the system what influence on a charge flow:

1. The charge pump has to supply enough flow to compensate for leaks due to the volumetric efficiency of the pump and motor. This is why sometimes the charge pump called a “replenishing pump”.

In general, volumetric efficiency of pump and motor in the hydrostatic transmission is around 96..97%, so system overall volumetric efficiency is around 93%. It means 7% of the theoretical pump flow is the leaks which need to be somehow compensated.

2. Addition flow needs for flushing of both main pump case and motor case to remove the hottest oil from the system.

Adding a hot oil shuttle valve at the motor removes the hottest oil in the loop through the motor case drain. Normal practice is to run the motor case back to the bottom drain port of the main pump then from the top pump’s drain port to the cooler (see the picture above).

There no ways to calculate exactly the required flushing flow through the motor. You need some experience with different applications and motor sizes, you have to do tests and check recommendations from the motor manufacturer. The generic rule to find the best flushing flow rate: motor should not be overheated in the worst-case mode.

3. Charge pump itself has huge internal leaks.

Mostly charge pump is a vane or gear type, and the volumetric efficiency of these type of pumps is very low, around 80%.

4. A small qty of flow is required to power the pump’s control system.

This qty of flow is pretty small.

Of course, different systems require different approaches to the calculation of charge pump displacement. And sometimes we have to make some compromises. If you have doubts – just ask a question!

Mounting flanges and shafts of pumps and motors

Just to determine what flange and shaft of pump or motor you have, please see below excerpts from the next standards:
SAE J744. “Hydraulic Pump and Motor Mounting and Drive Dimensions”.
ISO 3019-1. “Dimensions and identification code for mounting flanges and shaft ends of displacement pumps and motors”.

 

Two bolts mounting flange

Two bolts mounting

Two bolts mounting

Manifold for motors in close loop.

Just a note about one direction motors (not reverse!) in close loop applications.

There are a lot of different ways how to proceed with a hyd. motor in close loop applications to make motor work safe and reliable. From my experience, almost each one direction motor application is required to have the next items:

  1. Anti-cavitation check valve
  2. Pressure relief valve
  3. Case flushing valve
  4. Test ports

Each motor manufacturer has own solutions for their motors, but here are problems you can meet:

  1. OEM solution usually is not flexible: you can use only what they have. If you need for example flushing of the motor case – you have to design something external anyways.
  2. Price for these options is unreasonably high;
  3. Delivery time increased on 2..6 weeks;

Here is how I like to proceed for one direction motors – with an external manifold, mounted by hoses flange botls directly to both motor ports:

Motor Flushing

  1. The manifold is designed with through “A” and “B” SAE Code 62 ports with grooves for O-Rings from both sides. Using gender changer plates (for example, solution from Inserta) the manifold can be mounted in any direction/position and ports can be easy swapped if it required.
  2. Anti-cavitation valve is integrated to pressure relief valve is a very compact cartridge, which saves space in the manifold. Like an example, here are solutions from Hydac (DBRV16P) and Rexroth (MHDBN).
  3. Flushing flow control valves examples: HYDAC (SR06-01 or SR08-01) or SUN Hydraulics FDCB-LAN. The benefit of Hydac SR valves is SAE ORB cavity: if you do not need flushing valve you can still use this universal manifold but jut plug cavity with the regular ORB port plug.
  4. And of course test ports. I like to involve as many test ports as it is possible to make easy maintenance.

The benefit of using flushing flow control valve (vs fixed orifice) is you can adjust required flow for any motor size specificly for application conditions, you do not need to pick the size and replace the orifice with each new motor.

The only thing you have to be careful about is a motor case drain pressure. Therefore, before adjusting a flow put a pressure gauge between the manifold and port “D1” and increase flow slowly to reach the required flow rate but not exceed max case pressure.

Flushing line from the manifold needs to be connected to the bottom case drain port of motor. From the top case drain port, the line needs to be plumbed directly to a tank.