Forklift mast pump requirements and two pumps in parallel ??????

[xtal01]

I need a bit of help.

I am converting an old forklift into a lift. It is an old Raymond 20R30TN ( single mast extension, 3000 lbs lift capacity, 130" lift ).

I have basically scrapped 90% of the unit. I am only using the mast. I have even removed the "reach" unit from the lift carriage.

I now need to get some hydraulics on it. I was going to use the stock unit but it seems there would be a fair bit of work involved in building up a hydraulic package. I may do this in the end but for now I want something just to get the unit up and running.

What I am trying to figure out is the pump requirements.

The inside diameter of the cylinder is approximately 3". This equates to a piston area of about 7 square inches.

So, looking at the mast, I am guessing the cylinder must lift twice the listed weight ( each chain will have a load of 3000 lbs and the chain pulley must hold both chains .. thus 6000 lbs ... I have attached a picture, chain is fixed at one end and attaches to the carriage at the other .... pulley moves up with cylinder rod). Add say 1000 lbs for the mast and forks and we have 7000 lbs of lift.

This tells me I only need 1000 psi to lift the listed weight!

I think this is impossible .... 2000 makes more sense. Where am I making my mistake? Can you set me right.

I will never need this kind of pressure since the load I am planning to lift once or twice a day will only be 500 lbs at most .... but even then I want to make sure I am figuring the pump pressure correctly.

With a load of 500 lbs ( cylinder now lift 1000 lbs ) and a fork weight of 1000 lbs ... total load of 2000 lbs and a cylinder of 7 square inches, I get a pressure of only 286 psi! Again, I think I am way off!

I am looking at using a simple pump/tank/control unit from a dump trailer. 12 volts, complete self contained .... $400 new.

My next problem is flow. The largest (easily available and affordable) dump trailer unit I have found is rated at 2.0 gpm.

The cylinder has a volume of about 2 gallons. This means it will take 1 minute to lift the cylinder the entire height.

I want the lift time to be about half of this.

If I order two units, can I just "tee" them together and run them in parallel?

I will join the reservoirs together so they become one (so the oil coming back into the unit on the way down can split any way it want to).

I will put a check valve before each pump so the oil will not flow back through a unit if it is not running ( I can run one unit only if I want ).

Just wanted to make sure there is nothing I have missed.

Thanks .... Mike

 

[kiwitom235]

Hello,

All your calculations seem correct using your assumed values. cylinder pressure required is 69Bar for 3000lbs mass. so yes, the pressure is quite low.

Yes it should be fine to tee the two units together to double the flow. What size motor does the unit have on it, as it may be oversized as the pressure is quite low? You could install a larger pump on one unit rather than buying 2. Just a thought.

Also, consider a counterbalance valve as a safety precaution to stop the lift from suddenly dropping in case of hose burst etc.

 

[hydtools]

If you apply flow to both the head and rod end to lift, you will increase lift speed. The effective lift area will be the rod cross section. The required pressure will also increase. If the rod area is one half the piston area you will double the speed and required pressure. Lift in half the time.
Look up regeneration circuit.

Ted

 

[xtal01]

Very interesting ... I have never looked at a regeneration circuit.

Unfortunately this unit has a single acting cylinder with no "top port" so this will not work in this case ( thought I will keep it in mind for future projects ).

I do realize the "best" way would be a one large pump (possibly even a two stage unit) with a proportional flow control valve ( PLC controlled ). I have actually done this before for a customer who had a problem of "bouncing" his product when a hydraulic lift stopped. I started a zero flow .... ramped up to full flow then at 80% lift I began ramping down to near zero flow at the top.

Of coarse this was easy when your customer has an unlimited budget (I was doing instrumentation work in a plant that manufactured nuclear fuel).

This project is for my own home shop. The forklift only cost me $850. I was going to reuse the hydraulic pump off the unit but by the time I started pricing relief and control valves ... plus it is a 30 year old motor/pump (that seems to have been leaking for the past 10 years of it's life judging from the rags that were stuffed under the pump and in the base) I just think a new pump/motor/tank unit would be the most wise to use.

With the forklift, two pumps, batteries, chargers, and controls I should have $2500 to $3000 in the project.

The one nice part about using two pump is that I can cut one out just before hitting the top position. This will make it easier to hit the correct location.

I just can't find anyone who has tried connecting two pumps in parallel. I wanted to make sure I don't get any unexpected surprises.

Just FYI .... even though this is for my own shop, I do plan on installing a few safety items. I have a railing and chain for now but I want to install gates that are interlocked to the lift. This way you can not open a gate if the platform is not at that level. Also, a neighbor farmer actually tried to do a similar setup (he used a complete unit) and started a fire. He said it did not shut off. I was not there but my best guess is that either because of a fault in his controls or a fault in the contactor ( it may have welded on ), the motor ran till it or the wires overheated ( must have been a long time ). To solve this, I will install a timer to limit the maximum timer the pump will run. I will also put a second timer so that if the contactor does not drop out either a second contactor will open or I can "crowbar" an "expendable" contactor directly across the power lines and blow the main power fuse.

Thanks .... Mike

 

[kiwitom235]

2 pumps in parallel will be fine, just remember to install check valves on the exit of each pump. if you don't do this and you switch the motor off on one pump, it will turn in to a hydraulic motor!!!

 

[xtal01]

I ran into a problem ....

The guy selling me the "dump trailer" pump package did not realized there is a duty cycle of only 30 seconds. Also he over stated the flow!

Even with two of them joined together, it would take longer than 30 seconds to fill my cylinder!

So I am going back to building my own power unit.

I have the original pump of the forklift. I just need to add some controls.

I just put up a circuit in another post.

If you get a chance, can you check it out and see if it is correct .... already found one error on my downward flow control.

Thanks ..... Mike