Hydraulic motor question

[rangerssuck]

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.

[Jon Elson]

rangerssuck wrote:


> 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?

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

[rangerssuck]

Great. Thanks. Unfortunately, the idiots are likely long dead & buried, this machine is at least 50 years old. I'll have the electrician (quite a character, along with being smart) to get in touch with their hydraulics supplier and put together a pile of parts, including a shock absorber. He'll be able to explain to them what we're doing, and I expect they'l be able to recommend appropriate stuff.

But my main concern is whether this would work at all, and I appreciate your confirmation that it will.

[Gunner Asch]

On Thu, 26 May 2016 21:52:02 -0500, Jon Elson <el...@pico-systems.com>
wrote:

Typically the relief valve will take the shock of the motor shutting
down, and as the shuttle valve closes..the hydraulics are taken off
the motor. Any changes are due to sudden pressure change as the
system bleeds off through the relief valve and any leakage through the
motor

[Jim Wilkins]

"rangerssuck" <range...@gmail.com> wrote in message
news:66a72943-b550-49d6...@googlegroups.com...

======================
https://www.toro.com/~/media/Files/Toro/Commercial/education-technical-references/service-training-guides/09169sl.ashx?la=en

--jsw

[rangerssuck]

Thanks. I have instructed the plant manager to discuss the need for protection devices with their hydraulics supplier. I'm (pretty) sure they'll get it right.

[rangerssuck]

Though I think this problem is pretty much solved (ask me again in a week or so when I install the controls), this is a GREAT read. I will print it out and keep it in my reference library. It's much more clear than others I've seen, with illustrations of actual parts along with schematic symbols.

Thanks for the pointer.

[Ignoramus2195]

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

[Ignoramus2195]

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

[dca...@krl.org]

On Thursday, May 26, 2016 at 10:43:36 PM UTC-4, rangerssuck wrote:

>
> As always, all thoughts (especially kind ones) are welcome.

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 need 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 and 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 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 lock the position.

Dan

[Ned Simmons]

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

[Tim Wescott]

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
I'm looking for work! See my website if you're interested
http://www.wescottdesign.com

[rangerssuck]

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 close enough for the pins to go into their holes, everyone's going to be happy.

I am told by the machine operator who has been there 15 years that he has never seen it work reliably without playing with the microswitch position, and then it goes out of whack again after some (I'm not sure how much) running 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 that'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 need 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 hydraulic 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 recommends 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 valves and a couple of hours to hook it up.

[rangerssuck]

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.

[Jon Elson]

Ned Simmons wrote:


> I'd be reluctant to call the folks who designed the machine idiots.

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

[Jon Elson]

Jim Wilkins wrote:


>
> 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.

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

[Jon Elson]

rangerssuck wrote:


>> 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 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 lock the position.

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

[Jon Elson]

Tim Wescott 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.

I thought about this, too. First, wear could have made the whole system
loose. How does the hydraulic motor move the pallets? Chain or gear drive?
Any wear or looseness there?

Second, wear could have just reduced friction in the hydraulic pump and
motor so it coasts a lot longer than it used to.

Jon

[RogerN]

>"rangerssuck" wrote in message
>news:66a72943-b550-49d6...@googlegroups.com...
>
<snip>

>
>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.

Since they need to stop accurately, they probably had in mind switching
valves to stop the index instead of coasting to a stop. On the input side
of the motor you could have a 3-way valve that sends the pump back to the
tank and blocks the motor port at the same time. A valve to block the motor
OUT port should lock it up. For the hammer of sudden blockage, you could
use a relief valve or even have it switch to a restriction to ramp down to
position.

It's possible this already has the valves in the system and one isn't
working properly or sticking. We had a machine that used linear positioning
with a hydraulic cylinder, it had cartridge check valves and wouldn't
position correctly when the check valve was bad. Our larger machines have
hydraulic positioning but you proportional valves to ramp to position.

RogerN

[Jon Elson]

rangerssuck wrote:


> In position “B” (braking position):
> Connect pump line 1 to pump line 2

Yes.

> Connect motor line 1 to motor line 2

No, you want to block the motor ports, to stop the hydraulic motor.

A VERY simple on/off valve will do this.

Jon

[RogerN]

>"rangerssuck" wrote in message
>news:381649dd-8ffd-47ac...@googlegroups.com...
<snip>

>> 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 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 lock the position.
>>
>> Dan
>
>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.

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

[Gunner Asch]

On Fri, 27 May 2016 13:50:22 -0500, Jon Elson <jme...@wustl.edu>
wrote:

Well stated.

[Gunner Asch]

On Fri, 27 May 2016 10:27:31 -0500, Tim Wescott <t...@seemywebsite.com>
wrote:

VERY!! true.

[Tim Wescott]

I know I sound like I'm clinging to an idea, but -- maybe it broke
_before_ that operator got there? Lots can happen in 35 years.

Of course, the idea that wear has made it less accurate is quite
believable: one of the jobs of a _good_ control systems engineer is to
anticipate what happens as a machine ages, and compensate for it. Not
everyone does, or they figure that if it lasts for five years that it's
paid the bills and can be replaced or upgraded.

Maybe continue to use the microswitch for actuation, but connect it to a
solenoid valve? So hitting the switch simultaneously cuts power to the
motor and brakes the hydraulic system?

With the caveats about fluid hammer that have been mentioned by people
who know much more than I about hydraulics, of course.

It sounds like you just need to get the accuracy to the point where the
existing pins will engage, rather than +/- 0.001", which is nice.

[Jim Wilkins]

"rangerssuck" <range...@gmail.com> wrote in message
news:381649dd-8ffd-47ac...@googlegroups.com...

=======

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

[Ned Simmons]

On Fri, 27 May 2016 13:50:22 -0500, Jon Elson <jme...@wustl.edu>
wrote:

>Ned Simmons wrote:
>
>
>> I'd be reluctant to call the folks who designed the machine idiots.
>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.

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.

>
>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

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

--
Ned Simmons

[John B.]

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.

[rangerssuck]

That is certainly true. Unfortunately, the plant has been through a few owners over the years and documentation has been lax or lost entirely. I haven't seen this machine run, nor have I seen prints for it - I don't even know that they exist. When I get back there, I will, for sure, pull off a bunch of covers and trace out the hydraulic circuit. I really have no idea what I might find. It's possible that the work I plan to do has already been done and, as Roger suggested, there are stuck or leaky valves. We'll see next week.

> Of course, the idea that wear has made it less accurate is quite
> believable: one of the jobs of a _good_ control systems engineer is to
> anticipate what happens as a machine ages, and compensate for it. Not
> everyone does, or they figure that if it lasts for five years that it's
> paid the bills and can be replaced or upgraded.

Remember, this is a pre-PLC, pre-<anything modern> contraption. It is entirely possible that I'll find excessive wear in moving parts (chain drive, by the way) and will have to go back to the idea of an "almost there" sensor to trigger a slow approach and then a "there" sensor to stop. On the face of it, I have no problem with that, and it may be the easier way overall - all electronics, no oil. Right in my wheelhouse.

> Maybe continue to use the microswitch for actuation, but connect it to a
> solenoid valve? So hitting the switch simultaneously cuts power to the
> motor and brakes the hydraulic system?

I certainly could do that, but I don't know what the benefit would be to cutting the motor power. It's certainly easy enough to do, though, since the controls are already in place.

> With the caveats about fluid hammer that have been mentioned by people
> who know much more than I about hydraulics, of course.

And that's why I asked here. Even if you know next to nothing about hydraulics, you still know more than I do :-)

> It sounds like you just need to get the accuracy to the point where the
> existing pins will engage, rather than +/- 0.001", which is nice.

That appears to be correct.

[rangerssuck]

Well, that does make it a whole lot simpler, doesn't it?

[Tim Wescott]

That's certainly one way of making the thing robust in the face of wear.

Someone suggested a proportional hydraulic valve that's actuated by the
pallet position -- that would also work, but you might want to back it up
with your existing microswitch-and-off.

<snip>

>
> And that's why I asked here. Even if you know next to nothing about
> hydraulics, you still know more than I do :-)
>

That's really, really scary, given that most of what I know about
hydraulics is what you need to operate forklifts and fix the brakes on
your car.

[bruce2...@gmail.com]

Tim Wescott wrote:
>
> That's really, really scary, given that most of what I know about
> hydraulics is what you need to operate forklifts and fix the
> brakes on your car.

Me too, I might as well be driving a truck of oranges around right now. I don't know much about anything except what i've occasionally heard in construction (electrical, at that and driving trucks)