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My pressure calcuations are too low .... why ??????

My pressure calcuations are too low .... why ??????

My pressure calcuations are too low .... why ??????

So, I have most of the parts on the way for my hydraulic lift.

One problem keeps nagging at me.

The original forklift relief valve has a range of 600-3600 ( and the adjuster knob is cranked down fairly far ). On the forklift forum, the repair guys ( not a very helpful bunch when they find out you are modifying a forklift ) said the relief valve should be set to 2200 - 2500 psi.

This makes complete sense!

Then I do the calculations .....

At it's rated load of 3000 lbs, I keep coming up with only 750 psi !!!!!!!!!

Even adding 1000 lbs for the carriage I only get 1000 psi. I can't believe ( but may I am wrong ) they would set the relief pressure over twice as high as needed????

This just does not make any sense to me.

I keep trying to rethink the problem but I keep coming up with the same answer.

I have drawn a picture of the lift, the forces on the chain and cylinder and two examples of my calculations.

I have attached them to this post.

Can someone straighten me out on this. I am sure I am wrong but just don't see where.

Thanks .... Mike

RE: My pressure calcuations are too low .... why ??????

The load is the static load amount, but when the truck runs over even little bumps on the floor there can be an impact load that will be much higher. It would be unfortunate for the forks to drift down at every bump and require the operator to concentrate on maintaining the height as well as watch out for people and obstacles at the same time.

IOW it's one thing to lift the item, but another to go bouncing around while carrying the weight. It would not be good to pick up an item and break the machine with the first bump.

I don't design these, but I've seen how they are used and abused and think this is a decent line of reasoning.

RE: My pressure calcuations are too low .... why ??????

I agree on the "holding" ability of the lift. I would definitely want it to hold the load once lifted and I agree that a "bump" may put a load several times higher than needed to lift.

I can't believe they would give the machine the ability to lift over twice it's rated capacity. I just know there would be some guy out there in a warehouse that is going to try to lift 5000 lbs on a 3000 lb machine. I would think they would set the relief valve so that the machine will only lift it's capacity ... wouldn't they ?????


RE: My pressure calcuations are too low .... why ??????

If the system as designed can handle 2500 PSI, then there would be no reason to lower the setting. From a dynamic machine point of view, one on wheels that is, it is better to wind the relief in to keep its cracking pressure well above the maximum cylinder pressure as induced by dynamic load, when the machine hits a bump in the road etc...

Otherwise, the load would keep dropping and become more of a hazzard.

Your lift is only has one degree of freedom and shock loads are not to be expected, so the relief setting won't need to be so high.

just set it where you think it needs to be.

RE: My pressure calcuations are too low .... why ??????

In honesty, I am not really worried about raising the load ... I am worried about lowering it.

As some of you may remember, this lift project is just the mast off a forklift mounted as a fixed lift in a shop.

My average load will be around 500 lbs ... the carriage may weigh another 500 lbs so 1000 total.

This makes the system pressure only 250 psi (according to my calculations).

If this is correct, then valve sizing becomes important.

It seems different manufactures like to rate their valves at various pressures and flows.

Eaton rated on valve at 15 gpm ... but at this point it had a 250 psi pressure drop!

Parker rated a similar valve at 12 gpm .... but it only had a 70 psi pressure drop ( and only 150 psi @ 20 gpm ).

Anyway, I am sizing everything for 8 gpm flow and want to make sure I have a "reasonable" pressure drop through all valves at this point.

This is why I want to confirm my system pressure.

Thanks .... Mike

RE: My pressure calcuations are too low .... why ??????

What is your worry about lowering the load? If your system was adequate to raise the load, it should be adequate to lower the load.

If relief valve setting is too close to the system demand pressure, you have a chance to 'bump' into the cracking pressure of the valve which will open the valve a little and it will begin bypassing flow creating heat and reducing cylinder speed until your system demand pressure drops below the valve reseat pressure. In a full day of forklift operation the heat buildup would be significant and could trigger a machine over-temperature shut down.


RE: My pressure calcuations are too low .... why ??????


The relief is to cap the pressure, right?

This being the case, you don't need it to have full flow. It's only there to prevent damage to the cylinder.

Or, is it that you will be powering the lift down, effectively using the relief as a counter balance valve?

I'll look back at your other threads, remind myself of you system layout.

RE: My pressure calcuations are too low .... why ??????

OK...Lift cylinder is power up, gravity down.

Your oil flow out on the down stroke doesn't go through the relief, what are you worried about?

The relief valve is number 7 right?

RE: My pressure calcuations are too low .... why ??????

Sorry, I confused the discussion with two separate thoughts.

In my original post, I was thinking about the relief valve. Really, I guess this does not matter. If the original system had a valve set between 2200 and 2500, then setting my relief valve to say 2000 psi will protect everything ( all my new items are rated at at least 3000 psi).

My second thought was when lowering, if I have a system pressure of only 250 psi, do I need to worry about the pressure drop across the valves?

In the lowering circuit, I am going through three valves and a filter before getting to the tank.

If I am flowing 8 gpm through my first valve, I have a drop of 70 lbs @ 8 gpm ... so I now have 180 psi after the valve.

I now go through the flow valve .... 90 psi drop @ 8 gpm .... now 90 psi in system.

One more valve inline to the tank ... 100 psi drop @ 8 gpm .... so I won't even get 8 gpm through the valve ???????

Then I still have the spin on filter .... I am guessing 10 psi?

So I am just trying to make sure I don't need to "up size" my valves so that I get a lower pressure drop in order to flow 8 gpm through the system when lowering by gravity ( low system pressure with a "light" load ).

Heck, thinking along these lines, it may late forever to lower when it has no load on it!

Thanks .... Mike

RE: My pressure calcuations are too low .... why ??????

The return filter pressure drop should not be more than a few psi. You should choose one rated for 20 or more gpm. Most filters with bypass check valves bypass at 10 psi indicating needing replacement or oil that is too thick.

If the drop speed is too slow, open the flow control. Let the other flow restrictions be more significant in controlling the drop speed.


RE: My pressure calcuations are too low .... why ??????

Thanks once again .....

I was just going to use the same filter that came on the unit ( salvaged the spin on mounting, just going to get a new filter ... Baldwin BT292 was on it). I will check the flow on the filter first.

I was just worried that I would have so much pressure drop in the system that I could never get my 8 GPM.

So just for my own information, were my pressure calculations correct?

Also, has anyone used the Brand Hydraulics controller board EC0004A ?

I am a bit confused about the adjustments on the board .... I have sent 4 e-mails to the company and they have not answered.

There are 4 controls ( pots ) on board.

I have a PLC feeding 4 -20 mA into the board so I don't really want the board controlling ramp and such.

Ramp up and Ramp down .... I can set to minimum, instant response.

Minimum output ... again, set to zero as the PLC can control this and I want a zero output from the valve at some point.

Maximum output ... here I am confused. I assumed it was just setting the maximum flow output of the valve but the literature online talks about setting the maximum "speed"????? What speed... is this not the same thing the ramp functions do? Do they really mean the max flow?

Thanks ..... Mike

RE: My pressure calcuations are too low .... why ??????

You'll have to forgive me, I'm playing catch up a little...

The filter is the last thing line, before the tank, it will have the lowest pressure in the system. The filter will only normally see the oil coming out of the lift cylinder, when its lowering. The only other time would be when the relief valve spills, and that flow rate is not necessarily full pump flow. You need to make sure that the filter element is rated for the flow, even when clogged. Otherwise, unless it has a by-pass, it could cause trouble as it becomes more restrictive as the dirt content increases.

What is it that you are controlling proportionally...? There is nothing that is proportional control shown on the drawing.

I would ignore the speed remark, speed is only a function of flow..so yes, read it as flow.

RE: My pressure calcuations are too low .... why ??????

I have attached the original flow diagram.

The flow valve is a Brand Hydraulics EFC proportional flow control valve.

I do not "need" this to make the lift work. I am using it to get rid of the "bump" when starting and stopping the lift.

With the valve ( or actually the controller board for the valve ) being fed by a 4 - 20 mA signal a PLC, I can slowly start the lift moving ... accelerate to full speed, slow to a minimum speed and then "crawl" into the "up" position.

I am an instrumentation specialist so the PLC is easy for me.

I am also a machinist so building the unit has not been a challenge.

Hydraulics are me weak point. I have repaired a number of units but never really designed one.

I have been on this board asking questions ever few days for the last month or two. I just keep coming up with questions as I got through the design and living in Vermont, I have not been able to find any "local" experts on hydraulics.

I am sure I am overthinking this entire project but I just want to make sure I get it right.

The forklift mast is now mounted. The carriage is modified to take the forks directly ( the original unit had a "reaching" unit which I eliminated ).

Now I just need to get the hydraulics working.

This forum has been incredible!!!!!!!

Thanks again ..... Mike

RE: My pressure calcuations are too low .... why ??????

Right, all understood. Actually, I have been following your posts over the last number of weeks, I see what's going on.

Question now then is...what's stopping it working?

Is is ready to go?

RE: My pressure calcuations are too low .... why ??????

All that is stopping the work it me. I am just slow.

As I said, the physical mast is now mounted.

I ordered the flow control valve two weeks ago ... it came in.

I found a control board on ebay ... $100 rather than $580 new.

It came in. I hooked it to the valve and it worked.

I gave my list of valves to a local supplier over a week ago ... still no prices.

I checked the web sites and found distributors for both Eaton and Parker .... not sure if there is any other manufacturer I should look at. They are not "local" ( both in the next state ), but the "local" guy just seems not to have time for me (every time I call he says he is buried ... they are actually a truck repair shop .... no real hydraulic shops in the area).

I will send them my part numbers on Monday and get things going.

In my defense, I am still finishing up the house we just built. It took us 3 years ... lived in an RV on site while building. Not easy then winter nights can hit -30 but we saved $1000 a month in rent that we were paying. I just got a natural gas dryer hooked up this week ( had an electric one but it costing a lot to run ) .... I still have a lot of finish work that I can do over the winter.

I am hoping to get this project done in the next month or so.

When I get it done, I will post a You Tube video.

You guys are awesome!

Thanks .... Mike

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