Hydraulic motor question

A customer has a very large, very old (and pretty dirty) machine that needs some help. It is a multi station drilling and tapping machine. The product
ion items sit on 4" square pallets spaced roughly 8" center to center. Ther e is a microswitch that is activated by contact with the leading edge of a pallet. The switch causes the drive motor to stop, and the drilling & tappi ng to begin.
Their problem is that the stopping point is somewhat variable, and they tho ught that if I replace the microswitch with a prox sensor, all would be wel l. So, I went down there and had a look, and found a good place to mount th e prox sensor and a brush to sweep off whatever debris was there (and maybe an air jet for good luck). Then, I had a talk with the plant electrician.
The electrician, who really knows his stuff, pointed out that the controls stop the motor, but it's really a hydraulic motor that drives the system, a nd maybe it coasts a little when the electric motor is shut off. Also, he p ointed out, that the offset changes during the day, maybe as a result of th e fluid heating up. So, my not-at-all educated in hydraulic motors mind sta rted thinking, what if, instead of shutting of the pump, what if we install ed, right at the motor, the hydraulic equivalent of a DPDT relay that would disconnect the motor from the pump and put a short across the motor?
Does that make sense, would it (as would a DC motor) stop on a dime? I have googled a bit and I believe I have seen such circuits, but I'm really pret ty much in the dark about such stuff. If it DOES work, do I have to worry a bout overpressure in the lines? I haven't seen the machine run, but I'm tol d that a) it doesn't move fast and b) it doesn't move far.
The pump motor is 3-phase, so I suppose I could install a VFD (they probabl y have extras on site) and use a second prox sensor to cause the speed to r amp down, but it would be better if I could just stop the motor. And, there really isn't a good place to mount a magnetic brake.
As always, all thoughts (especially kind ones) are welcome.
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rangerssuck wrote:

So, they have a single-purpose motor that runs a pump, that runs the hydraulic motor? Geez, what idiots, of COURSE it will coast with all that inertia.
Yes, you want a solenoid valve that relieves the pump and blocks the motor. It may need to have some kind of shock absorber to prevent a hydraulic hammer from wrecking the hoses. I'd check with a local hydraulic provider for recommended parts, this is a fairly common job to do. it ought to work quite well and index the pallets much more accurately.
Jon
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On Thursday, May 26, 2016 at 10:52:05 PM UTC-4, Jon Elson wrote:

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Great. Thanks. Unfortunately, the idiots are likely long dead & buried, thi s machine is at least 50 years old. I'll have the electrician (quite a char acter, along with being smart) to get in touch with their hydraulics suppli er and put together a pile of parts, including a shock absorber. He'll be a ble to explain to them what we're doing, and I expect they'l be able to rec ommend appropriate stuff.
But my main concern is whether this would work at all, and I appreciate you r confirmation that it will.
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On Thu, 26 May 2016 20:16:36 -0700 (PDT), rangerssuck

I'd be reluctant to call the folks who designed the machine idiots. I can't tell you how many times I've worked on a piece of old machinery that seemed like a kluge at first glance only to end up with an appreciation of what these guys were able to do with what they had available to them.
The old school method of insuring that an indexing table without a positive drive (e.g., the cam indexers Iggy mentioned) is a "shot pin;" a pin or dog that pulls the table into the precise desired position once the table stops. Are you sure there wasn't one on this machine, or perhaps there still is and it's not working or worn out?
You can buy purpose-built guided shot-pin cylinders https://www.phdinc.com/product/?product=clamps&series=psp http://www.btmcorp.com/shot-pin-cylinders.html
Two concerns re the hydraulics. First is, you're right that you can't just slam the motor ports closed. You need some sort of decel device and a bypass for the pump output. But you're still faced with the fact that the oil viscosity will change thru the day, and those devices will be sensitive to that change. The motor also has internal leakage, also sensitive to viscosity. And wear, and increased internal leakage, in the motor may explain some of the drift you're seeing.
My other concern is whether you can find a hydraulics salesman who can afford to take the time to really solve your problem. Thirty years ago the hydraulics houses around here had enough staff that they could dig into a problem. They still have some sharp guys, but unless it's a big job, they just don't have the time to fully analyze a problem.
--
Ned Simmons

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Ned Simmons wrote:

Normally, yes. But, if the only way to stop the pallet is by shutting off an AC motor and letting the whole system coast to a stop, AND it is required that the pallet index reliably, then I'm sorry, but this was a bad design. Certainly, if no precision was needed, it would be fine. If precision indexing was desired, then even in 1960, the technology was available.
You could use a shot pin, as you say, or a hydraulic valve that would stop the hydraulic motor much more rapidly than the coasting motor. Or, a Geneva mechanism to advance the pallets and index them, and a small coast of the hydraulic motor would not cause the pallet to move. So, there were lost of WELL-KNOWN techniques that were in widespread use much more than 50 years ago.
Jon
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wrote:

It's a big leap from recognizing that the design isn't currently ideal to claiming that someone's an idiot. Seems to me to conclude that you'd have assume that the machine is being used as it was first conceived; that there haven't been changes since it was built; and that the OP's explanation of the machine is complete & correct. The first two are very unlikely if the machine is indeed over 50 years old. The OP has added some info in later posts that belies the third.

Later posts indicate there is a shot pin or similar device on the table.
--
Ned Simmons

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A customer has a very large, very old (and pretty dirty) machine that needs some help. It is a multi station drilling and tapping machine. The production items sit on 4" square pallets spaced roughly 8" center to center. There is a microswitch that is activated by contact with the leading edge of a pallet. The switch causes the drive motor to stop, and the drilling & tapping to begin.
Their problem is that the stopping point is somewhat variable, and they thought that if I replace the microswitch with a prox sensor, all would be well. So, I went down there and had a look, and found a good place to mount the prox sensor and a brush to sweep off whatever debris was there (and maybe an air jet for good luck). Then, I had a talk with the plant electrician.
The electrician, who really knows his stuff, pointed out that the controls stop the motor, but it's really a hydraulic motor that drives the system, and maybe it coasts a little when the electric motor is shut off. Also, he pointed out, that the offset changes during the day, maybe as a result of the fluid heating up. So, my not-at-all educated in hydraulic motors mind started thinking, what if, instead of shutting of the pump, what if we installed, right at the motor, the hydraulic equivalent of a DPDT relay that would disconnect the motor from the pump and put a short across the motor?
Does that make sense, would it (as would a DC motor) stop on a dime? I have googled a bit and I believe I have seen such circuits, but I'm really pretty much in the dark about such stuff. If it DOES work, do I have to worry about overpressure in the lines? I haven't seen the machine run, but I'm told that a) it doesn't move fast and b) it doesn't move far.
The pump motor is 3-phase, so I suppose I could install a VFD (they probably have extras on site) and use a second prox sensor to cause the speed to ramp down, but it would be better if I could just stop the motor. And, there really isn't a good place to mount a magnetic brake.
As always, all thoughts (especially kind ones) are welcome.
=====================https://www.toro.com/~/media/Files/Toro/Commercial/education-technical-references/service-training-guides/09169sl.ashx?la=en
--jsw
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On Friday, May 27, 2016 at 6:59:46 AM UTC-4, Jim Wilkins wrote:

ferences/service-training-guides/09169sl.ashx?la=en

Though I think this problem is pretty much solved (ask me again in a week o r so when I install the controls), this is a GREAT read. I will print it ou t and keep it in my reference library. It's much more clear than others I'v e seen, with illustrations of actual parts along with schematic symbols.
Thanks for the pointer.
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Jim Wilkins wrote:

VFDs can deliver both dynamic and DC injection braking to the motor. If you wanted to avoid any change to the hydraulics, you could set up a little PLC or even some relay logic. Add a sensor for "near", and have the VFD slow the pump motor to, say, 10 % when the near sensor trips, then command stop when the index sensor trips. This should get much more precise indexing without touching the hydraulics. A bit of tweaking with the positions of the sensors and the VFD programming should get you at LEAST a 10 X improvement.
Jon
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If you want a rotating table to stop precisely, it is called indexing. Tell them to buy an indexing table, like by Camco or whatever. They are designed to stop exactly where they are supposed to stop and are adjustable with cams or whatnot. Some of them are pneumatically operated. I sold a bunch of them in the past.
Compared to the cost of screwing around with electrics, hydraulics, etc it is easier to spend a few grand on a proper solution.
i
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Almost forgot
Shameless plug: I just bought two huge rotary drilling stations in auction.
http://www.bidspotter.com/en-us/auction-catalogues/premier-asset-recovery-group/catalogue-id-bscprem10013/lot-f3458f2f-a7cb-4603-8617-a601016081b7
http://www.bidspotter.com/en-us/auction-catalogues/premier-asset-recovery-group/catalogue-id-bscprem10013/lot-b5935076-cb3d-453c-95ad-a601016081b7
They are both rather big, and have a bunch of controls, a Bridgeport head on each, one has a PanelView 550, etc.
The point is that there is a rotating table underneath each machine, let me know if you may be interested.
i
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On Thursday, May 26, 2016 at 10:43:36 PM UTC-4, rangerssuck wrote:

Not solutions, but maaybe a kind thought.
I would post this over in SED. Much more up that alley than RCM.
And I realize that you are just starting to think about the problem, but do you have any idea of what are the requirements? How repeatable does it ne ed to be. And how accurate? Obviously it has been used as is, but what is desired.
If you want it really precise, then you will need a way to close the loop a nd have feedback from the position. But just getting close may be all that is needed.
My own thought would be to add a valve and line. The valve would open when the pump is commanded to stop moving and would connect the pressure side t o the return side. So even if the motor kept running , there would be no p ressure to the hydraulic pump. May be a lousy idea as it would not lock the position.
Dan
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On Friday, May 27, 2016 at 9:51:29 AM UTC-4, snipped-for-privacy@krl.org wrote:

do you have any idea of what are the requirements? How repeatable does it need to be. And how accurate? Obviously it has been used as is, but wha t is desired.

and have feedback from the position. But just getting close may be all th at is needed.

en the pump is commanded to stop moving and would connect the pressure side to the return side. So even if the motor kept running , there would be no pressure to the hydraulic pump. May be a lousy idea as it would not loc k the position.

I'm feeling like taking your approach one step further and connecting the t wo lines from the motor to each other should get the thing to stop on a dim e. But see my response to Tim for more detail.
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rangerssuck wrote:

No, connecting pressure to return of the MOTOR would allow it to coast to any position. Right now, the pump acts as a bit of a brake on the motor.
A traditional relieving valve would allow the pump to be unloaded, but would block the ports on the motor, holding it in position. This is a standard valve type.
Jon
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<snip>

That works on electric motors but on hydraulic motors the fluid out would be free to flow to the IN, allowing a coast to stop. If you stop the flow in and block the flow out it will stop instantly but have a high pressure spike due to inertia. A relief valve could relieve the high pressure spike or you can ramp down using flow control of some sort.
RogerN
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wrote:

I'm feeling like taking your approach one step further and connecting the two lines from the motor to each other should get the thing to stop on a dime. But see my response to Tim for more detail. =====Maybe you could add a normally-closed solenoid valve tripped in advance, in series with an easily accessible throttling valve, to bypass enough of the flow around the hydraulic motor to slow it down before it's stopped.
You could tie it into other solutions, like releasing the valve to help stop the motor, then opening it to free the motor when the locating pin actuates.
If it doesn't work you can just close the valve and leave it there.
--jsw
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On Fri, 27 May 2016 08:55:50 -0700 (PDT), rangerssuck

There was a hydraulic system used on aircraft, and probably in other places, that used a so called "open center" system that worked as you describe. See: http://tinyurl.com/gw652cq
--
cheers,

John B.
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On Thu, 26 May 2016 19:43:33 -0700, rangerssuck wrote:

Before I did anything else I would ask both myself and any old-timers if the machine used to work better. It might even be worth a visit to the retirement home of a former millwright or two.
Then I would check for signs that something on the table is there but broken, or has been removed.
It may be that the only thing the "idiots" who designed it did wrong were to retire and leave it in the hands of _real_ idiots.
The only way you're going to get the thing to have precision in line with modern machine tools is to either have some positive closed-loop control of position, no doubt with ball screws and motors and whatnot involved, or to have some fixed positive indexing (like the "shot pin" mentioned elsewhere).
Deciding to revamp it with electronic controls may just be jumping onto a slippery slope leading to replacing the whole damned thing.
For the fixed indexing, you probably want a pin or whatnot to establish the correct location and then something that holds tension against the pin (else the whole pallet will be short of the pin, and your precision will have gone out the window).
Sometimes the quickest and most reliable super-high-performance modification to an older machine is to put it back the way it was originally designed.
--
Tim Wescott
Control systems, embedded software and circuit design
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On Friday, May 27, 2016 at 11:27:39 AM UTC-4, Tim Wescott wrote:

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OK - in no particular order: Iggy's idea of an indexing table would be fine except that it would require rebuilding the entire machine which is about 20 feet long, six feed wide and six feet high. It simply isn't happening.
There is pin indexing, but the current stopping position is so bad that the pins are totally misaligned with the holes. If I can just get it to stop c lose enough for the pins to go into their holes, everyone's going to be hap py.
I am told by the machine operator who has been there 15 years that he has n ever seen it work reliably without playing with the microswitch position, a nd then it goes out of whack again after some (I'm not sure how much) runni ng time. I AM sure that he told me the overshoot is worse in the afternoon than it is in the morning, and he and the electrician are pretty certain th at's due to the oil temperature.
Of course there could be any number of other things wrong - there could be leaky valves, worn pump, worn motor, sticky relays, who knows what? But I n eed to start somewhere, and this looks like a pretty simple thing to set up .
For test purposes, all we have to do is hook up the hydraulics - the hydrau lic equivalent of a 4PDT switch thusly: In position ?A? (normal running position) : Connect pump line 1 to motor line 1 Connect pump line 2 to motor line 2 In position ?B? (braking position): Connect pump line 1 to pump line 2 Connect motor line 1 to motor line 2 along with whatever relief valves or shock absorbers the supplier recommend s and hook it up to the existing microswitch (probably through a relay). If it works, great. If not, it will have cost a couple of bucks for the valve s and a couple of hours to hook it up.
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rangerssuck wrote:

Yes.

No, you want to block the motor ports, to stop the hydraulic motor.
A VERY simple on/off valve will do this.
Jon
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