hydraulic circuit review please

[xtal01]

Hello,

I am re-purposing the mast/cylinder and pump from an old electric forklift as a stationary lift.

I am an instrumentation designer / technician and don't usually get into hydraulic circuits ( just the electrical end ).

Can someone review my design. I would appreciate any advice or comments.

There is actually two designs. They are almost identical. The only difference is that one uses a 3 position 4 port valve and the other a check valve and 2 position 2 port valve.

The 3 position valve give two ways of stopping the lift ( valve and motor ) and better approximates what was there originally ( a manual spool valve that had a switch on it to start the motor).

The 2 position valve and check valve is how most simple ( cheap ) dump trailers are built.

Thanks so much ....... Mike

 

[kiwitom235]

OK are the flow control valves pressure compensated priority flow? if this is a single acting cylinder (gravity down) then you dont need a priority flow fr the down stroke, just a standard needle valve or pressure compensated 2 port will do. see here for flow control valves:

http://www.parker.com/literature/Literature Files/IHD/FCsection.pdf

and you only really need a 2 position valve not 3 position.

 

[xtal01]

Thanks so much for giving it a look.

Yes, the flow valves are pressure compensated priority flow. Because this is a "home project" I was planning on using a "cheap" valve. Northern tool sells Northman ..... FC 51 ... $90

I thought I needed the three position valve ( center closed ) to "hold" the cylinder in position ( if I went with the circuit on the "right" then I only need a two position since I have a check valve to hold in the "up" position ).

I actually want to change the flow while the lift is in motion, this is why the flow valve on both the "up" and "down". I will do this with a mechanical linkage attached to the carriage to "trip" the flow valves.

I am doing this to get rid of the "bump" when starting and stopping the carriage ( it will only have an two positions ... full up and full down).

I realize this is a very odd way of doing things ... but it is considerably cheaper that using a proportional valve with an interface board and a PLC. I actually did this once for a client who had a "large" budget. If I remember, I spent about $3K on parts for the project.

Since this is a lift for myself, the budget is much tighter. The forklift I salved for the project only cost me $850. I figure the entire project will come in at about $3K.

Thanks ..... Mike

 

[xtal01]

Huge mistake on my part ....

The factory engineer just got back to me and suggested ( I think as you did ) that the "down" will not work.

The flow in would be equal to the flow out ( ports EX plus CF ). He suggested plugging the EX ( the excess flow port ) or just putting in a simple ball valve.

Did I miss anything else???

Thanks ..... Mike

 

[hydtools]

I think you should provide a way to dump the pump to tank when your directional valve is centered or off. Even if the motor is switched on and off with the directional valve, there is a coast down time when in your circuits the pump/motor will be driving flow through the relief valve at pressure. That would create drive line stress you can or should avoid.

Ted

 

[xtal01]

Interesting .....

On a forklift forum, it was suggested ( they did not see my circuit as I could no put a picture on my posting ) that I do not start the motor under load.

So, if I am reading both of you guys correctly, I should put a "tee" just after the motor and connect a normally open valve to dump back into the tank.

When I signal my control valve to move, then I will also put power to the "dump" valve.

Does this sound reasonable?

Thanks again .... Mike

 

[hydtools]

Use an open center spool in your three position valve. This will connect your pump to tank when centered.
Or install a pump unloading valve in the outlet line of the pump. This will automatically dump the pump.
Turn the pump on before operating the system. Turn the pump off when not operating the system.

Ted

 

[xtal01]

When I looked at an open center spool valve, I thought that all 4 ports now connect together when in the center position.

If this is true, then the cylinder will drain and come down when in the center position rather than stay up. This is why I chose the closed center option.

I will check ( and maybe you can tell me ) is there an option for some valves to join ports P and T but block A and B ?

Do they make ( and again I will do some homework tonight ), an automatic valve ( I am guessing it would sense differential pressure or flow and close when the system is at rest) or do I do this with a simple solenoid valve?

I truly appreciate all the help!

Mike

 

[hydtools]

Open center cylinder spool: in the center position P is connected to T; A and B are both blocked. Also known as a tandem center valve.

A pump unloading valve senses downstream pressure. It opens the pump outlet to tank at a setable pressure. A check valve in the line to the P port of the directional valve maintains the sensed pressure to keep the pump bypassing until either the directional valve is opened or you turn off the pump.

Ted

 

[xtal01]

Wow ..... I can't tell you how much I truly appreciate you help.

As I said, I have done a number of jobs but never had the chance to work with hydraulics. It is interesting, I worked for a custom air compressor shop some years ago. We did things very similar ... unloading before starting, unloading and idling large compressors ( and not shutting them off ) when air was not called for.

I do know enough to check with experts before trying to build a project.

I am in VT ... very rural. VT's largest city is only 42,000. Industry is minimal up here. It has been very had to find any "experts" in the are.

I tried a few local forklift and tractor repair shops. They can tell me how to repair items but not how or why systems are designed.

I truly appreciate you help.

I will do a lot more reading this weekend and take another shot at the circuit.

Thanks again .... Mike