On Thu, 26 May 2016 20:16:36 -0700 (PDT), rangerssuck
wrote:
On Thursday, May 26, 2016 at 10:52:05 PM UTC-4, Jon Elson wrote:
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
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.
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/?prod...mps&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