Hydraulic Car Ramps...... need to make them go up and down

[Dukeman]

TL;DR Summary

Currently I have to jack the ramps from under the car... no fun.

I currently jack up the end of the ramps using the channel frame and a 3 1/2 ton jack, because of the height of the channel frame I have to have it sitting quite far back under the car for clearance when the ramp is lowered down to drive off.

I have to push the jack a long way under the car to lift up the end. I want to make it easier... cheaper the better as usual. But I am looking at two hydraulic cylinders mounted under the ramps, one each side and use them to lower an arm with a wheel on the underside. This will lift up the ramps high enough to place the support frames under the ends.

I have looked at 2 x hydraulic cylinders...
32mm bore
20mm rod
200mm stroke.

Would these be strong enough? Or would the rods bend?
I have NO idea how to work out how strong the cylinders would need to be.
Also would like to connect them to a hydraulic hand pump or air/ hydraulic pump.

I had a good making a cardboard example to see how it could work.

Any ideas please?

 

[Tom.G]

It looks like you will be retracting the hydraulic cylinders to raise the car.

That gives an effective piston area of π x (16² - 10²) (that's piston - rod areas)
or 3.14 x (256-100) = 3.14 x 156 = 490mm², or 4.9cm², or .76in².

From the cardboard mockup, when the leg is folded parallel to ramp:
Mechanical DISadvantage is:
cylinder pulling on leg to raise ramp: 7.5 (pulling almost across the leg pivot)
tip of leg to cylinder attach relative to leg length: 5.3
= 7.5 x 5.3 = 39.75 Mechanical DISadvantage

weight on each leg, assume 1000lbs or 454kg
cylinder force needed: 39.75 x 1000 = 39 750lbs, or 39.75 x 450 = 18 047kg
hydraulic pressure needed: 52 300psi, or 3 683kg cm^-2

Conclusion: not possible with current or foreseeable technology.

Oh well; nice thought though!

Cheers,
Tom

 

[Ranger Mike]

we used an air cylinder to lift up the race car to change tires
https://www.mcmaster.com/air-cylinders/tie-rod-air-cylinders/tie-rod-air-cylinders-7/
Shop air compressor was good enough to power it. you may need a couple of them. cost is pretty cheap.

 

[Dukeman]

Hi Tom,
Thanks for your reply, what if I turn the whole thing around so the cylinder is pushing to lift the car?
I did not think about the reduction in force due to the area of the cylinder rod... assuming I am on track?
I have no experience with hydraulic power.

Hi Mike, with pneumatic cylinders are the pushing straight down?

 

[Ranger Mike]

you can mount them in many ways. Note the two holes in photo. These are on top of the mount blocks connected by the tie rods. You screw in an air fitting in either hole to push or pull the cylinder. real easy to mount and fabricate.

 

[Dukeman]

Just to add - I only want to lift and lower the one end the the car drives on to, the front will pivot.

Lift the rear end, remove 2 x supports, lower the rear end and back down and off the ramps.

I do not think pneumatic would have enough power, worked with them for years, but if the hydraulic cylinder pushed the arm down and lifted the ramp... could that work?

I will mock up a sample out of cardboard...

 

[Dukeman]

Righty Ho... here is another option using the cylinder to push.
Remember this is upside down and it would be mounted to the underside of the ramp and there would be a wheel on the end of the arm which will push on the ground and lift the ramp.

 

[Tom.G]

what if I turn the whole thing around so the cylinder is pushing to lift the car?
I did not think about the reduction in force due to the area of the cylinder rod... assuming I am on track?

Yes. You would then need only 70% of the pressure calculated in my post #2.

https://www.physicsforums.com/attachments/img_1626-jpg.262614/
This change, moving the cylinder attachment point further down the leg means you need only 34% of the calculated pressure. Combine the two options for 0.7 x 0.34 = 0.24 (24%) of the pressure calculated in post #2.
12 550psi or 884kg cm^-2

Still not practical though. The highest hydraulic pressure I've heard of in use was 10 000psi on a military aircraft; and it took them an extra year or so to get that to work.

There are higher pressures in use (30 000psi and up) for Water Jet cutting of hard materials, but the safety concerns are extreme. A quote from one site on Water Jets: Waterjet cutting is used to cut through extremely hard materials and is fully capable of removing a limb from an inattentive operator.

Even air at 100psi can be dangerous enough. Here in California, Industrial Safety Laws limit the pressure in an air gun nozzle to 30psi when used on an assembly line. (The work-around here has been to put a pressure regulator at each workbench but have a connector before the regulator "For Use By Authorized Personnel Only" )(the 10 000psi hydraulic and 30psi air numbers are several years old and may have changed by now)

The approach by @Ranger Mike (https://www.physicsforums.com/posts/6339717) seems the best bet so far. I suggest you investigate it further (even if you can't use those hydraulic cylinders you already have! :).

Cheers,
Tom

 

[Dukeman]

HI Tom... good thing I have not purchased any cylinders yet, really appreciate your help. So larger cylinders could work? I just need to know what size I would need?

Would pneumatic cylinders have enough power?
Only wanting to raise and lower one end of the ramps.
My 3.5 ton jack can lift both.

I need clearance once the ramps have been lowered, so I can't have cylinders sticking up. I could have a cylinder on the outer sides of the ramps, nothing under unless it is low profile.

 

[Ranger Mike]

how much your car weigh? ifin you go to mcmaster carr they have specs on how much each cylinder can lift .. 3 1/2 inch broe over 700 lbs at 100 psi air pressure and you have additional mecahnical advantage via linkage

 

[Tom.G]

Would pneumatic cylinders have enough power?

Yes. It's all in the sizing (and mechanical design).

My suggestion would be to follow @Ranger Mike 's approach, mount the cylinders on the outside edge of the ramps with the rods pointing straight down. This will give the most lifting force at the lowest cost.

Before you get to the cylinder lifts, figure out a way to have some sturdy legs on the ramps that AUTOMATICALLY come down to support the maximum weight when the lift is raised. These must be able to hold up the ramp without any air pressure. They can literally be a lifesaver when an air hose bursts!

You will need to fabricate some strong, secure mounting brackets. If the ramps shift sideways under any condition, it will collapse; possibly bending the piston rods in the process.

You will also need some wheels on the end of the piston rods. This is to accommodate the slight movement towards the hinged end of the ramp when raised. Make the wheels large enough (diameter and width) so they do not deform or damage the flooring (even if it's concrete). It may be advantageous if you can figure a way to allow the wheels to tilt sideways slightly, thus accomodating an uneven floor. (haven't thought this one through though)

Low cost air compressors, at least here in the USA, will only get to about 60psi so perhaps the cylinders should be sized to lift the vehicle and ramps at 50psi. That way you can use most any compressor you can find and be able to upgrade to a better one when/if you replace the car with a heavier one.

To choose the minimum size cylinders, take the weight of the car and divide it by the assumed 50psi air pressure, that will give you the required piston area. Divide the area by pi (3.14) and take the square-root of the answer. That gives the piston Radius, so double the Radius to get the piston Diameter and look it up in a catalog. (since there will be 2 cylinders, this gives the typical engineering 2x derating)

Next you must decide how high a lift you want. Remember to account for the mounting configuration of the cylinders to the ramp. Now add an inch or three to get the piston stroke.

Air cylinders require lubrication. This is done by mixing oil into the compressed air using something called an oiler. Don't forget it, the cylinders will rapidly wearout if ignored.

Those are the basics but BE AWARE THERE IS LITTLE SAFETY FACTOR BUILTIN. For completely non-critical applications, like hauling a rock on a truck on private property with no consequences if it falls off, a safety factor of 2 is often considered acceptable. Where a life is threatened by a failure, a safety factor of 10 or more is often recommended, sometimes with an additional backup added.

If this is a private project for your own use only, you get to value your life -- or a missing limb. Otherwise get a professional engineer and a lawyer before proceeding. I suggest you print this post as it may get removed for being dangerous... even though it answers you question on how to size air cylinders, there were other important items added.

Cheers,
Tom

 

[Dukeman]

Thanks - Noted.
Cheers!

 

[Lnewqban]

Summary:: Currently I have to jack the ramps from under the car... no fun.

I have to push the jack a long way under the car to lift up the end. I want to make it easier... cheaper the better as usual.
...
Any ideas please?

Good advice above.
Have you made any progress?

This is a detailed example of a properly designed pure hydraulic lift:
https://www.quickjack.de/media/wysiwyg/Downloads/3 - QuickJack Manual - Feb 2018 -Revision.pdf

This 800 watts motor can lift a regular car in 30 seconds.
The power unit, hoses, fittings and hydraulic rams make the system expensive, even if you fabricate the rest.

The cheaper solution depends on what you have at your shop: compressed air, capability to weld and cut steel, machining, etc.

Some additional suggestions:
1) Relocate the bar/bridge towards the end of the ramp and fabricate some brackets that allow you to pull with bridge from above the ramps.
Example:
https://www.kwik-lift.com/photos/

2) Replace floor jack with air/hydraulic bottle jack that can be remotely controlled.
Example:
https://www.harborfreight.com/12-ton-air-hydraulic-bottle-jack-94487.html

3) Use pneumatic bladder jack. One 3-ton could replace current trolley jack. Using one 3-way manifold, you could feed and control one smaller bladder under each ramp.
Example:
https://www.amazon.com/dp/B07V5WKW52/?tag=pfamazon01-20

4) Going purely electric. Expensive if buying the whole thing. The good thing about these manufacturers is that they sell independent parts that you can adapt to your particular needs.
Examples:
https://www.autoliftproduction.com/en/autolift3000

https://www.ezcarlift.com/ezcarlift_video.html

For general safety features of car lifts:
https://www.bendpak.com/best-car-lift-guide/car-lift-safety/

 

[256bits]

his 800 watts motor can lift a regular car in 30 seconds.
The power unit, hoses, fittings and hydraulic rams make the system expensive, even if you fabricate the rest.

So what is in that valve unit to ensure that the car lifts evenly on each side? ie level lifting.

 

[Dukeman]

I all - thanks for the advice.
I am looking at using an airbag.
I have found quite a few that will fit perfectly, I will bolt the lifting frame to the air bag.



I am pricing one from China -
https://www.alibaba.com/product-det...etails.deiletai6.8.6ad332ebsQpZmR&bypass=true
min order 10 but I have asked for 1 units... fingers crossed.

 

[Lnewqban]

So what is in that valve unit to ensure that the car lifts evenly on each side? ie level lifting.

I have no idea.

They could be using a hydraulic divider valve that keeps both flows equal regardless differences in pressures.
Please, find "spool-type flow divider for 50-50 split" in following article:

https://www.hydraulicspneumatics.co...42/chapter-13-flow-controls-and-flow-dividers

Perhaps they have tested the thing enough to decide that it works sufficiently well on the Pascal's principle, avoiding complexity and additional costs.

Good question!