Changing direction of a ram

[clem dog]

Right, if I have a hydraulic power pack and a double acting hydraulic ram and I want it to change direction automatically at the end of each stroke, forwards and back, continually how would I do it?
I don’t really want to involve electrics in it, just a spool valve of sorts

Oh and as is obligatory I want to do this as cheaply as possible

Any ideas

Thanks Clem

 

[peterd51]

Hi,

I picked up a Lego kit at a charity shop the other day, builds a bulldozer or a crane and uses pnuematics...only cost £3!

Anyhow, there's a small finger-operated pump that feeds into a couple of three-position switches. Middle position is off so you can choose which ram you're moving, flip the switches one way and it pushes the attached ram out, flip it the other way and it pushes the ram in, I'm sure you get the idea.

So how about uprating this to a full size set up for hydraulics where a lever attached to the end of the ram flips a valve back and forth as it reaches its minimum and maximum points? Possibly position the valve in the middle of the stroke with a spring on each side? Or maybe have it wire operated so you have some 'fine' adjustment?

Regards
Peter

 

[Woody.v8]

Could you not just use a hand operated spool valve and reverse the ram as needed? Maybe you could bias the spool on the the return stoke for safety?

 

[wyn]

It might be possible to use the movement of the ram to move the spool lever at the end of each stroke using some sort of mechanism.

The only trouble is the lever would have to go through the neutral position to get the ram going in the opposite direction...so it couldn't just be a linkage.

It might need something like a weight acting on the lever that could be pushed by the ram at the end of it's travel, which would then be enough to take the spool lever through the neutral position. Or maybe something spring loaded.

But there again that could all be rubbish.

 

[chriskelland]

Have a look at a Weir pump.

Chris

 

[Drains]

Clem, I think this is one for Snowcat. In the meantime, take a look at http://www.harrieronline.co.uk/

If you go to the hydraulic cartridge valves section, then sequence valves, you'll see the kind of thing that will get you to where you want, but it won't be totally simple. The sequence valve will detect the increase in pressure caused when the ram gets to its full extent. It will then open and push fluid to another stage. However, what you may need is a sequence valve in conjunction with something else, where the sequence valve acts to open/ reverse an auto spool valve or a cetop valve (but i'm guessing, so don't quote me!)
Hydraulics is complicated, but can be much simplified by the suppliers of the valves, as they can fit them to operate in ways and sequences to your spec.

If you don't get satisfaction here, phone Harrier and have a chat. They know their stuff, and their products are reasonably priced.

Also, check out "pilot operated check valves" - could be the thing?

 

[scarecrow]

I did see something like that on a log splitter on you-tube when the ram went to the end of its stroke is automatically returned until it was triggered again
I don't think a lever would work because the ram would stop when the valve reached the neutral position maybe if you had a hydraulic ram working the spool in some way that one triggered the other

 

[Mikey D]

I know on pneumatic cylindrers you can get a reed switch which attaches to the outside of the cylinder and its activated by a magnetic strip on the piston , . not sure wether you can get them with hydraulics , that would involve electrics tho .
cheers mike

 

[clem dog]

Thanks for your replys lads, that seems to be the way I was thinking about this. An ordinary spool valve has to go through neutral to change flow, so this would leave a dead spot where nothing would happen.

So if a spool valve that was sprung to both sides had a lever actuated by a ram moving it, it would spring to the other side of the valve. This would be ideal although i cannot find any such thing on the net.

Si, those valves really look trick! The other thing on the Harriers website was a plough overturning valve which im not sure about. These work on the of a ram on a reversible plough. When the spool valve is pulled the ram retracts, when the end of the stroke is reached it then extends the ram without pushing the spool valve the other way. Hope that both makes sense and is correct.

So im not sure if this changes direction of the ram because of a pressure difference as the end of the stroke is reached or if it is because of back pressure through the valve as the plough falls over centre. Also i know that a plough will not continue to keep turning over and over as long as the spool is left open, the spool must be release and then opened for this to happen again.

Thinking out loud now

Thanks

Clem

 

[snowcat]

hmmm, what sort of speed for the changeover???? the check blocks used on ploughs work by dumping the pressure in the cycle, they don't change flow, gravity of the beam pulls the ram over, strange things as they can ex-cert flow either way......

I think your going to have to cycle through a mechanical or electronic interlock method , whats it for maybe a mechanical cam, could achieve the desired effect?

the easiest method I could think is with a solenoid valve and a limit switch.......Id have to think about the wireing tho.... not cheep mind

 

[clem dog]

This is for making a passage scraper for a cubicle house so i guess speed of changeover is not that critical, just so long as it does!

Never really figured on a cam/crank type system, thought it may involve too much maintenance with bearings needing greased and so on. Also thought to have a wheel with a driving rod on it would have to be of to large a diameter to allow somewhere in the region of three feet of linear movement. But i suppose it may work out much cheaper in the long run.

Would it be possible to just use a diverter valve to change direction or would this have a neutral spot in the middle where nothing would happen.

I dont really want to go down that route of elctrics because them and sh1t dont really mix that well also did not want to spend too much but if needs must!

Would you know an aprox. cost for that snowcat?

Thanks

Clem

 

[snowcat]

Had a think about this and a 3 way valve will have a dead point to make it fail safe....

The simplest method I can think of is to use a solenoid changeover valve and a couple of limit switches with N/C and N/O contacts wired in series so one cancels as the over is tripped, not ideal with muck involved, but I wonder if a nudge bar of sorts could be made to run linner with the ram the bar could be encased in a protective housing of sorts... interestingly it is possible to buy limit switches just for ram apps, these are fail safe so if a switch sticks it still cant power up two open contacts....

This circit would continually cycle so an Estop with a set of normally open contacts should also be wired in serries.....

a second way would be to have a set of hydraulically activated valves, this would be quite complex to plumb, the other trouble is there would be no easy way of making an estop on the circit unless you through the main pump, I haven't also seen a valve that would replicate the role of an electric limit switch...

Cost wise I would think under £200 for the electric version, wouldn't have a clue TBH on the hydraulic activated circuit..

Chain and flight scraper maybe?

 

[Justme]

How about using a standard hydraulic valve block but set up a spring loaded lever on the ram so that in the last couple of inches of travel it compresses the spring & then in the last 1/2" its releases the spring pressure to move the lever through the dead spot in to the return position? Repeat for the return stroke. Simple & cheap.

 

[clem dog]

Ok just to say thanks for all your help on this.

As I was looking into this, I came across a power unit for an automatic scraper at an auction so that sorted all my problems as it had the proper valve on it already along with a ram and some of the track needed to allow the scraper to run!

Now all i need to do is make some extra track to fit the full length of the cubicle house but thats a project on its own!

Thanks

Clem