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Change hydraulic motors in rotary unit for multipurpose drilling rig

Change hydraulic motors in rotary unit for multipurpose drilling rig

Change hydraulic motors in rotary unit for multipurpose drilling rig

At my work we have just purchased a drilling rig that has a hydraulic powered rotary unit. The unit has two hydraulic motors, that can be run in series for high speed low torque mode, and in parallell for low speed high torque mode attached to the same shaft. The high speed is used for coring, and the low speed is used to get everything moving, and for higher torque applications like DTH hammer. There is a valve that selects how the oil flows in respect to series and parallel. In addition to the two hydraulic gears there is also a mechanical reduction gear for real low speed high torque, but for coring the gear ratio is 1:1.

The motors used on the unit are Danfos OMS80 motors with PVG32 propotional valves. Due to the motors being limited to 80 lpm - and 950 rpm at max speed the flow of the circut is limited to 80 lpm@280 bars pressure.
I would like to increase the torque output of the rotary unit at the required speed - that is 900 - 1000 rpm for coring with NQ - wireline. My idea is to change the spool valve to the biggest in the PVG32 family - 130 lpm, and then also change the hydraulic motors to larger motors, around 120 - 130 lpm motors, so that the larger flow and displacement will give more flow at the same speed of the rotary unit. The flow needs to increase to keep the speed of the motors, and the bigger displacement will give more torque at the same max pressure. The prime mover and the hydraulic pump on the circut will be able to support the higher flow at the same pressure. Cooling should also be adequate.

Would bent axis fixed displacement piston motors be a good idea to use as hydraulic motors in this case? I have found motors like Sunfab SCM130 that should fit if we make adapter plates and use spline adapters. My reasoning, as a geotechnical engineer, not a mechanical engineer is that a bent axis motor should be more efficient than the orbital motors, especially at 1000 rpm, and that bigger motors should give more torque.
Is there generaly any problem of running bent axis motors in series? Paralell should not be any problem at all, but I am a bit more concerned about series. In my head a orbital motor should have more leakage than a bent axis motor so that cavitation should not be a problem. As far as I can understand the two motors will share the 280 bars of pressure at max, and that the first motor will se 140 bars of back pressure. Sunfab said that their motors should take 150 bars, but one will need a case drain, that will be present.

I also have some questions about mounting flanges and shafts, but those are other details.

RE: Change hydraulic motors in rotary unit for multipurpose drilling rig

You will be increasing torque by ~50%. No idea if the gears can handle this, depends on your duty cycle and gear safety factor. Even if you don't break them you could get into pitting, which may be offset by using a thicker/better lube. And high power will create more gearbox heat.

Also need to determine if the motor drive end bearing is used to hold one side of the input gear. If so the increased torque will increase the separating forces, so you need to make sure the replacement motor can handle ~150% of the OMS80.

Upsizing the proportional valve from 80 to 130lpm could greatly diminish speed control for DTH operation. You didn't mention what motor speed you typically need for DTH. You want the bit carbides to hit fresh rock with each hammer blow so the correct speed setting is important to maximize penetration rate and bit life. Could also look into a dual valve set-up.


RE: Change hydraulic motors in rotary unit for multipurpose drilling rig

Thanks for your reply. The rotary unit is delivered with a lot of options for larger motors than the OMS80 motors that are mounted now. Up to 315 ccm motors can be fitted from the factory, so the torque should not be a problem. In the case of drilling with DTH we will probably use larger motors like OMS250 for speed control. The Power for the gearbox might be a problem, that we will have to look into.

As far as I know the motors will not hold the bearings, but that is something that I will look into.

My main concern is to find out if there is a large difference in running pistonmotors in series vs running OMS motors in series.


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