When I enable the circuit that powers the VFDs and servos, I'm finding that the main MCB and/or RCD trip. This seems to have been an intermittent feature of the hydraulic pump for some time now, as far as I could tell. But now I can't even get it to power up regardless. What's going on here - time to get to the bottom of it.
I reckon it's likely to be the pump motor. And access to the VFD wiring is a bit of a PITA, so perhaps the thing to do is simply remove the hydraulic "power pack" and see what's up - assuming that's there the problem is. Besides, I'd like to have a closer look at this part of the machine. (More later!!)
The front fixings for the oil tank are easily accessible. I hope TF there aren't any rear fixings, as this thing is tight against the wall and weighs several tonnes (thought to be around 3).
By stretching my arm to the back, I can see where any rear screws would be. Can't see anything, as it's too dark. Getting in to that area would be an absolute nightmare. The machine frame is solid cast iron with only a few apertures, mostly internal at that.
If I remove that adjustable foot bolt thing, perhaps the tank will pull clear?
Yes! Indeed, there's nothing holding it in at the back, so clearly there are no rear fixings - TFFT! Shouldn't need any, as it's not likely to walk on its own.
Regardless of whether there's anything wrong with it, let's have a look in more detail.
Here's the hydraulic pump block that is responsible for the operation of the turret and tailstock (I don't use the collet chuck). It's a Sperry Vickers H1J PVB6A-RS-20-0A-11 Can I find any specs for this?
Let me just think about that price for a few seconds. Ok, done, now move on - I'm probably not going to cough up £2k for one of these. Besides, mine works fine, even if I think it's a bit noisy (and what would I know, that may be normal).
That's driven by a fairly straightforward 3-phase motor rated at 2hp / 1.5kW / 230V. It's 4 pole, so would run at 1800rpm in the US but 1500rpm in the UK, where it's been running fine - I set up the turret timing with the way it worked in this installation. It's pretty large by today's standards, given its limited power output but hey.
It seems to be a variable displacement pump, so presumably it regulates the displacement in order to regulate the output pressure. In this application, it spends most of its time simply providing a static head.
"The Sperry Vickers (Eaton) part number PVB6A-RS-20-0A-11 designates a variable displacement, inline axial piston hydraulic pump delivering a rated flow of 6 GPM (22.7 L/min) at 1800 RPM." That sounds like a swash plate type pump. I was expecting a "gerotor" type pump but this makes more sense, as it will avoid roasting the oil by recirculating it against a relief valve at full flow rate.
The PVB6A-RS-20-CA-11 pump belongs to the PVB Piston Pumps series and is produced by Vickers for use in hydraulic automation circuits where controlled flow is required. It is an axial piston, variable-displacement unit, so delivered volume adjusts to system demand and reduces wasted input power. Clockwise shaft rotation aligns with common right-hand drive layouts found on mobile and industrial machinery. A geometric displacement of 13.81 cm³ per revolution establishes the fundamental output capacity, while a rated flow of 22.7 L/min is achieved at 1800 rpm for moderate actuator speeds. The rotating group tolerates a maximum shaft speed of 1800 rpm, allowing direct coupling to standard four-pole electric motors without gear reduction. Pressure regulation is handled by a pressure compensator control that trims displacement as the outlet approaches the 140 bar limit, preventing overload. System piping connects through 11/16-12 UNF-2B SAE O-ring main ports, providing reliable sealing for high-pressure service.
The swashplate operates on one side of center, giving a single-direction displacement zone that simplifies motion control logic in closed-loop circuits. Case drainage is routed through a 9/16-18 UNF-2B SAE O-ring port to keep housing pressure low and protect shaft seals. Suction conditions must remain under 1.0 bar absolute inlet pressure to avoid cavitation during high-speed operation. The pump is specified for ambient temperatures from −20 °C to +70 °C, accommodating both cold-start field equipment and warm factory floors. An operating viscosity window of 220 to 13 cSt is permissible, with a recommended range of 54 to 13 cSt offering optimum efficiency and reduced wear. Power transmission to the drive train uses a 4.8 mm square by 25.4 mm long shaft key, ensuring positive torque transfer to standard couplings without custom hardware.
Technical Specifications
Case Drain Port Thread
9/16-18 UNF-2B (SAE O-ring)
Control Type
Pressure Compensator (C)
Displacement Zone
One Side Of Center (S)
Geometric Displacement
13.81 cm³/rev (0.84 in³)
Main Port Thread
11/16-12 UNF-2B (SAE O-ring)
Max Ambient Temperature
+70 C
Max Inlet Pressure
1.0 bar (15 psi)
Max Outlet Pressure
140 bar (2000 psi)
Max Shaft Speed
1800 rpm
Min Ambient Temperature
-20 C
Pump Type
Axial Piston (Variable Displ.)
Rated Flow @1800 Rpm
22.7 L/min (6 gpm)
Shaft Key Size
4.8 mm sq x 25.4 mm long
Shaft Rotation
Clockwise (R)
Viscosity Range (operating)
220 to 13 cSt
Viscosity Range (recommended)
54 to 13 cSt
Anyway, enough of that. The pump's probably fine, so I need to just focus on the motor itself. First show that (if) the motor itself is the issue that is causing the RCD and MCBs to trip. I don't imagine I need anything like 1.5kW just to operate the turret....
The pump is driven by an Oldham coupling and of course, given this motor's parentage and vintage, it has a 1" shaft. That could be a royal PITA.
The motor itself is foot mounted, with a flange mount for the pump assembly. So if the motor is shagged, I'll need a foot and flange motor with imperial measurements. Enough doom scrolling - let's disconnect the motor and see if the fault has gone away - and the Yaskawa drive isn't buggered.
Good news and bad:
Turns out there's nothing wrong with the pump motor, it seems. Disconnected the motor entirely and the breakers trip when I close the switch to the VFDs and servos. I guess I can now start to disconnect the various branches from that node and see where the fault lies......
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