Math conversion question / Sizing velocity fuse

[jtbinvalrico]

I'm trying to size a pair of velocity fuses for my scissor lift install. The manual indicates the following data regarding the hydraulic side of things. There's a master cylinder in one scissor, and a slave in the other.

Flow Rate: 2.1 cm3/g
Continuous Working Pressure: 240 bar
Peak Pressure: 250 bar

Seems one piece of valuable information to have would be the normal flow rate, which is listed as 2.1cm3/g.....My google-fu is failing me and I can't find a conversion for this to gallons per minute. It does seem that it's a multi-step conversion; but maybe it isn't. Trying to figure this out is migraine-inducing....

a) Anybody out there who can convert 2.1cm3/g to GPM?

b) Feel free to chime in with any hydraulics advice on sizing these velocity fuses. I read on one vendor site that the idea is to size it so that it clamps at 30% over the normal flow....I have no idea.

Thanks for any help. Here's a link to the scissor lift install thread:

https://www.garagejournal.com/forum/showthread.php?t=383717

 

[rcktsled]

Are you sure about that cm3/g? Could it be cubic centimeters /second? Flow rate should b in some form of volume/time.

 

[rcktsled]

I just looked at the PDF manual for your Atlas lift. It does say cm3/g. I'm not familiar with that unit. Hydraulic pumps are sometimes rated in displacement per revolution. I don't know what "g" represents. Sorry

 

[LX-Markham]

g is a metric gram (weight). So that makes no sense. It’s like reverse density.

Must be a typo. Should be ‘s’

1 cm^3/s = 0.0159 us liquid gallon / minute

 

[jtbinvalrico]

Asking Google to convert 2.1 gram / cubic centimeter to anything yields results such as this:
280.40559078 oz/gal (ounce / US gallon)
Problem seems to be that it's missing the "time" element as I try to get to GPM. Perhaps there's some sort of assumptions or standards that come into play in getting that number to mean something "per minute." Wish I had studied more in school...

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[MattT]

Took a look at the lift manual and found out your 2.1cc/g is the pump spec. Pumps are typically spec'd in ccs per rev so "g" is likely a cut and paste from a foreign language, or maybe a typo.

Then I took a look at their motor specs and it shows 2,800 rpm which is either 50Hz data or a really crappy motor

So using 3,400 rpm for 60Hz operation it crunches out at 1.886 gpm. That's just a best guess based on the OEMs **** documentation.

 

[greg13]

Call the manufacturer, let them figure it out.

 

[matt_i]

Here's a typical hydraulic velocity fuse for use in a hydraulic "manifold". Its just adjustable to a "gpm flow rate" before the spring is tripped and the orifice closes. 9 liters/min is the lowest one this will go via classic spring adjustment.

http://www.eaton.com/ecm/groups/public/@pub/@eaton/@hyd/documents/content/pct_274134.pdf

Hopefully you have some machining capability to create the ports for it in a (usually) block of aluminum or could be cast iron or steel. Its not a standard pipe thread.

 

[jtbinvalrico]

Took a look at the lift manual and found out your 2.1cc/g is the pump spec. Pumps are typically spec'd in ccs per rev so "g" is likely a cut and paste from a foreign language, or maybe a typo.

Then I took a look at their motor specs and it shows 2,800 rpm which is either 50Hz data or a really crappy motor

So using 3,400 rpm for 60Hz operation it crunches out at 1.886 gpm. That's just a best guess based on the OEMs **** documentation.

That's a tip in the right direction. Thanks.

Call the manufacturer, let them figure it out.

Called, got transferred, left a message. No reply.....Sent an email, no reply.

Here's a typical hydraulic velocity fuse for use in a hydraulic "manifold". Its just adjustable to a "gpm flow rate" before the spring is tripped and the orifice closes. 9 liters/min is the lowest one this will go via classic spring adjustment.

http://www.eaton.com/ecm/groups/public/@pub/@eaton/@hyd/documents/content/pct_274134.pdf

Hopefully you have some machining capability to create the ports for it in a (usually) block of aluminum or could be cast iron or steel. Its not a standard pipe thread.

Some of the guys who've done this have managed to find the necessary adapters to get them to fit properly. My main concern is sizing them correctly so they'll do their job if needed.

Thanks all for the input.

 

[jtbinvalrico]

Following MattT's lead....

Poking around on some Asian websites for hydraulic pump specs, I saw that this expression was commonly used: cm3/rev

I then put the word "revolution" through a simplified google Chinese translator and the result was 革命 Gémìng.....quite possible that this is where our "g" came from.

So: Flow Rate = 2.1cm3/rev

Google says 2.1cm3 = .0005547613 gallons

rev = 2800 rpm

2800 x .0005547613 = 1.55 GPM

Close? Does this figure sound plausible? If not precise, does it pass the smell test and get us in the ballpark?

For what it's worth, google says cm3/g is a valid expression when discussing heart valves and chemistry

 

[Fix Until Broke]

Another approach is to work this backwards.

How fast does it lift now? (looks like 60 sec in the manual)
How fast does it fall now?
How big is the cylinder (bore diameter, rod diameter, stroke length)?

Typically the cylinder extends to raise and retracts to lower, so calculate the volume of oil in the head side of the cylinder (area of bore x used cylinder stroke).

Let's say (for example only) that it's a 3" cylinder that strokes 34" from full raise to full lower. The cylinder has an area of 7.07 square inches and strokes 34 inches for a volume of 240 cubic inches. There are 231 cubic inches in a gallon so this is just a bit over 1 gallon of volume (convenient ). If it retracts in 20 seconds, then you'll want something a bit bigger than 3 GPM so you don't get nuisance trips of the flow fuse when you lower it.

Probably something in the 4 GPM range will work well in this example. Probably cost ~$100 from a local distributor for the cartridge, manifold (body) to mount it in, plumbing etc.

https://www.sunhydraulics.com/model/FQCA

Here's an article on flow fuses in general - https://www.hydraulicspneumatics.co...draulic-fuses-add-safety-and-control-circuits

They're doing some slightly unusual things in the hydraulic circuit and I'm assuming that the two cylinders are not sized the same given how the circuit is laid out.

I'm guessing that cylinder 1 rod area is equal to cylinder 2 head area and that valve #4 is typically closed during normal operation (only opened to "level" the two cylinders).

Ideally, the reason that flow fuses are not typically used is that there are much better solutions in the form of counterbalance valves, sometimes known as overcenter valves. These mount directly to the port of the cylinder and require the cylinder to be pushed down (no gravity lower). It's powered up and powered down - if a hose breaks the cylinder(s) stop moving instead of continuing to fall. Unfortunately it does not look like this type of circuit is used as this would require a directional valve that powers the cylinders down instead of just lowering - this is likely all driven by cost.

Sorry for the long reply - hope this helps.

 

[jtbinvalrico]

Sorry for the long reply - hope this helps.

....I appreciate the long reply!

This is the kind of information I can put into my build thread, which is geared toward the "highly motivated novice" It really helps to understand the thinking and process behind the inner workings of these things....and that acts as a check against our work.

I'll wait until I've got the scissors up and running before adding these fuses. That'll let me take these measurements, check my math, confirm fuse size, and add this info to the thread.

Thanks again.

 

[Fix Until Broke]

One thing I forgot - the two flow fuses won't be of the same flow rate due to the area difference in the cylinders.

Cylinder 1 will be a higher flow rate than cylinder 2

 

[MattT]

I then put the word "revolution" through a simplified google Chinese translator and the result was 革命 Gémìng.....quite possible that this is where our "g" came from.

I'd say that makes "g" definitely rev. Good find

rev = 2800 rpm

Like I said earlier that 2,800 rpm number is probably 50 Hz data which is why I used 3,400 rpm, for 60 Hz operation, in my calc. Other than that your math is good.

That said looking at the schematic Fix Until Broke posted the lower is gravity not powered so the flowrate may be higher. So I agree with him that the best way to figure this is to calculate the cylinder volumes and time the lower with the lift heavily loaded.

 

[matt_i]

Ideally, the reason that flow fuses are not typically used is that there are much better solutions in the form of counterbalance valves, sometimes known as overcenter valves. These mount directly to the port of the cylinder and require the cylinder to be pushed down (no gravity lower). It's powered up and powered down - if a hose breaks the cylinder(s) stop moving instead of continuing to fall. Unfortunately it does not look like this type of circuit is used as this would require a directional valve that powers the cylinders down instead of just lowering - this is likely all driven by cost.

In the world of pneumatics there's a "pilot operated check" which more or less traps air within the cylinder, so even if there's a total loss of main line air pressure nothing moves. I'd have to look up an ANSI schematic but in basics the UP signal which would put air in the bottom of a cylinder also must open the check valve on the DOWN-port before air can be expelled, and vice-versa. There is the fail mode of the cylinder seal bypassing but usually that's a faraway event and it starts gradually displaying leakdown instead of being a catastrophic fail. I don't have enough experience to know if the same logic element is available for hydraulics.

 

[Fix Until Broke]

There's PO checks in the hydraulic world, however they're notoriously unstable unless you have a separate, independent pilot pressure source (which this circuit does not have).

If you can't get the cylinder measurements, you can extend the cylinder on an unloaded platform, block it up, disconnect the hose on that cylinder only and then lower it while catching the fluid in a bucket to then measure the volume.

 

[jtbinvalrico]

Another approach is to work this backwards.

How fast does it lift now? (looks like 60 sec in the manual)
How fast does it fall now?
How big is the cylinder (bore diameter, rod diameter, stroke length)?

Typically the cylinder extends to raise and retracts to lower, so calculate the volume of oil in the head side of the cylinder (area of bore x used cylinder stroke).

I've got everything up and running and I was able to take these measurements. I'm learning a lot about hydraulics and my rusty math. Here goes:

Lift time from zero to max height is 60 seconds.
Fall time unloaded is 78 seconds.

Master cylinder: Bore diameter is 5.2", rod diameter is 2.62", stroke length is 20.5"

Slave cylinder: Bore diameter is 4.49", rod diameter is 1.97", stroke length is 20.5"

Master cylinder volume = (pi) x (bore radius squared) x (stroke length) or 3.14 x (2.6 x 2.6) x 20.5 = 435.14 cubic inches. If there's 231 cubic inches in a gallon, the master cylinder volume is 1.88 gallons. If it takes 78 seconds to retract from full height, the master cylinder has a flow rate of 1.44 gpm....unloaded. Should this be a loaded calculation?

Slave cylinder volume = same equation, 3.14 x (2.245 x 2.245) x 20.5 = 324.43 cubic inches. One gallon is 231 cubic inches, so the slave cylinder volume is 1.40 gallons. At 78 seconds to retract, the slave cylinder has a flow rate of 1.07 gpm.

And.....(love learning new stuff) Fix Until Broke notes that the rod side of the master should equal the head side of the slave in this situation:

Rod side of master = Volume of the cylinder as calculated above, minus the volume of the rod itself. Rod volume = 3.14 x (1.31 x 1.31) x 20.5 = 110.46 cubic inches. Total volume of master 435.14 ci - rod volume 110.46 = 324.68 cubic inches.....I got 324.43 ci for the head side of the slave.

Thanks to Fix Until Broke for the really cool hydraulics lesson!

So the master is 1.44 gpm and the slave is 1.07 gpm. Allowing room for nuisance trips, would a 4 gpm fuse work? Same for both? 4 gpm for the master and 3 gpm for the slave? Should I redo all these calculations with a loaded fall rate?

 

[Fix Until Broke]

If you have the ability to perform a loaded test, that will help you insure the fuses are sized correctly. Valve 12 in the schematic is a "pressure compensated flow control". This valve is set to only allow a certain amount of flow through it, regardless of the pressure applied (works like this)

I'd guess that it's set to fall at about the same rate as it raises (~60 seconds), so if you can easily test it, that's the best way to confirm. Whatever this flow rate ends up being, you can use this to size the flow controls. I'd set them at least 25% larger than this flow rate (33% is a safe place to start) for each cylinder.

In the end you don't want the thing to "drop like a rock" if a hose fails before the fuse trips, but also want to avoid nuisance trips.

If you get some nuisance trips, insure all the air is purged out before condemning the flow fuses. A shot glass sized bubble of air will wreak havoc on those - the gas expands very rapidly and will trip the flow fuse (doing it's job). Once all the air is purged out the hydraulic decompression is relatively small and shouldn't trip the fuse.

Glad to help - keep us posted on how it works out!

 

[Oilguy]

That is a size 16 cartridge which is "huge" for a small scissor lift. Eaton lists cartridge body numbers and options on that page you posted. They do that for each cartridge. The size 16 is standard size used by most manufacturers of cartridge valves except Sun, which has their own thread.
Excuse me. That is a size 12. Still to large.

 

[Fix Until Broke]

A flow fuse won't care if it's in an oversized cartridge - more cost/size/weight/etc, but very little functional issues in most of the designs.

In standard cavity sizes, a -8 or -10 will be the typical "standard" cavity sizes that you'll find. In "Sun" this will be a Series 0, 1 or 2 with Series 1 being the most common for the flow rates you'll be looking at.

 

[jtbinvalrico]

I timed a fully loaded descent using my F150. To be consistent, I timed it from the second highest lock (about 69") down to the lowering cutoff. The platforms stop a few inches above the floor and require an additional press of the button to lower into the pits. This does not represent the entire up and down travel of the platforms, but the truck would only go so high before hitting the ceiling. I timed the same distance unloaded.

Loaded: 45 seconds
Unloaded: 55 seconds

To relate this to a real-world scenario with a fall from a full height of 71" and a full descent past the lowering cutoff into the pits, I bumped my loaded descent time up to 49 seconds.

Master cylinder: Contains 1.88 gal of oil and empties under load in 49 sec

GPM for the master cylinder = (1.88 gal X 60 sec)/49 sec = 2.30 gpm

Size velocity fuse at 33% over gpm = 2.30 gpm X 1.33 = 3.06 gpm

Slave cylinder: Contains 1.40 gal of oil and empties under load in 49 sec

GPM for the slave cylinder = (1.40 gal X 60 sec)/49 sec = 1.71 gpm

Size velocity fuse at 33% over gpm = 1.71 gpm X 1.33 = 2.28 gpm

The Vonberg website lists 28000-204 and 28000-206 series velocity fuses. The 28000-204 fuse comes in a flow range of .1 to 4.0 gpm, while the 28000-206 fuse comes in a flow range of .5 to 10.0 gpm.

https://vonberg.com/products/view/51

I'm thinking I should use one size for the master cylinder and a lesser size for the slave cylinder as suggested. If I need a 3.06 gpm and a 2.28 gpm, I'll need to find out what increments these are made in. Perhaps size the 3.06 up to 3.10, and the 2.28 up to 2.30.....Maybe the 28000-204 fuses are sized in .1 gpm increments and the 28000-206 fuses are sized in .5 gpm increments. Gonna call them and report back.

 

[Fix Until Broke]

Assuming your plumbing is all 1/4" NPT, I'd get two of the same series at the flow rates you've calculated above. The plumbing/adapters to get to/from SAE to NPT and back will be trouble enough, not to mention having to increase/decrease size as well.

Plumbing might dictate what brand/cartridge/body you use more than anything else. Best advice I can give is to adapt away from NPT as soon as possible. Right at the cylinder port would be best. Going with -4 or -6 ORS will be the easiest to plumb/adapt/configure by a long shot. You'll hate NPT by the time you're done

 

[jtbinvalrico]

You'll hate NPT by the time you're done

Noted....Greg Smith is generously replacing the leaking master cylinder post warranty. All my hose ends are 1/4 BSPP. The parts diagram indicates a washer between a 1/4" lower port fitting and the cylinder. I'd assume that's BSPP as well.

So I'll try to go from 1/4 BSPP (with a face seal?) from the cylinder to SAE to accommodate the velocity fuse. That would leave getting the SAE end of the velocity fuse to play well with the BSPP end of the hose. I've identified some Parker adapters that should cover that end.

Thanks!

 

[jtbinvalrico]

I just don't understand why you should hold all these numbers in your head. When I do all these calculations - I simply use calculator they gave me in the university (one of these: https://bestcalculators.net/best-graphing-calculator-for-college-reviews/). And by the way, why your size velocity fuse is only 33% over gpm? Is it always like this?

One of the problems with simply plugging numbers into a calculator and not being aware of the math and process behind them is that you lose a valuable check on your work. If you're a pro, you know when a value is out of the norm; a novice has no such reference and benefits from understanding why the values are what they are.

Why 33%? First, someone with more knowledge than I told me that was a proper number. Second, learning the process and understanding why you want some headroom, but not too much, has led me to regard that number as appropriate.

I know more about hydraulics now than when I started this thread because I'm willing to be educated by the guys on this board. [emoji481]

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