Torque for Mechanical "scissor" jack 3

[aspearin1]

Hello All:
I'm looking for typical torque values for lifting a load with the average mechanical "scissor" jack, most commonly found in the trunk of a car (center horizontal drive screw and hinged joints). I'd like to fit an electric gearmotor to this jack in direct drive, but I need to estimate the required torque to lift my load. Has anyone come across some curves plotting torque vs. load for various jack types? Any equations for this type of estimation? Thanks in advance.

ChemE, M.E. EIT
"The only constant in life is change." -Bruce Lee

 

[MintJulep]

Test it. Then you'll know what you need for your application.

 

[aspearin1]

Yeah, I know... That was plan "B".

ChemE, M.E. EIT
"The only constant in life is change." -Bruce Lee

 

[GregLocock]

Use a work calculation (W*z=T*theta), then multiply the estimated torque by 4 for friction.

Cheers

Greg Locock

 

[ivymike]

yeah - what greg said.

 

[CESSNA1]

ASPERING1:

Test it. The friction coefficients can vary all over the place. I have a jack that is loosem the gets tight and then loosens again as it raises. They are cheaply made and fits and tolerances vary all over ther place. Not to mention the cleanliness of the threads

Regards
Dave

 

[aspearin1]

W=work
z=height
T= torque
Theta=? (pitch of the threads? angle of the brackets?) Forgive me if these questions are too basic. I'm more of a chemical guy.

ChemE, M.E. EIT
"The only constant in life is change." -Bruce Lee

 

[GregLocock]

The rotation of the torque, in radians, if you are in sensible units.



Cheers

Greg Locock

 

[johnwm]

Two 'real world' answers:

1. They are designed for little old ladies to pick up the corner of a car. Typical handle 6" long, typical old lady will pull 20lbs. Torque = 40 ft lbs to lift 500 lbs

2. They make caravan (trailer) jack operators from battery screwdrivers plus a bit of hex bar and a socket spanner

Good Luck
johnwm
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[JStephen]

Lifting a normal car, probably never over 100-ft-lbs, usually less than half that. Based on guessing the force required. You can use an 8" ratchet wrench on some of these, and still get enough leverage to raise okay. Based on guessing the force required. Usually lug bolts require more torque than the jack. It would be pretty simple to do the test- just get a torque wrench in the right range and use a jack that has a socket for the handle attachment.

Be careful motorizing one of these things. They are lubricated with a heavy grease, but they aren't really made to be used a bunch of times. Maybe 10 times in the car's life? Anyway, if you start running that jack up and down with a motor, I think that grease is going to disappear pretty quick.

I think if you'll look at the geometry, torque is highest for a given lift when the jack is all the way down. But lifting cars, even if you start with the jack all the way down, you have to raise it a ways before you get the full weight on the jack. So you practically never operate one of these jacks in the maximum-torque condition.

 

[GregLocock]

JStephen raises an excellent point. In at least two different companies I have worked for they have been designed for a very small number of cycles. More than 10. Slightly.

Cheers

Greg Locock

 

[aspearin1]

Thanks for all the input. Given the life cycle you point out, perhaps hydraulics or pneumatics will be a better choice for my machine. With my current design, the jack would be under constant load of around 100 lbs, and at times up to 300 lbs in its extended position. This would occur at least 30 times in a given day, likely more. Thanks again.

ChemE, M.E. EIT
"The only constant in life is change." -Bruce Lee

 

[GregLocock]

If you make the screw thread and hinges properly there is no reason why it shouldn't last that long. You might want to look at a two post hoist in a garage for inspiration, some use chains and chainwheels, others use a vertical leadscrew. A leadscrew is nice since it can be made failsafe very easy, in case of loss of power.



Cheers

Greg Locock

 

[zekeman]

I'm sure you can buy an offtheshelf lift that will meet your
life requirements, WITHOUT redesigning the mechanism. Your requirement is very minimal.

Before you buy, however check out the hinge area and the thread detail for good design as Greg implies.

By the way, given all the cars that are junked, I would think that there must be plenty around in junk yards that would easily work Most have rarely or never been used.

 

[dcme2]

I'm currently working on a senior design project that involoves a scissor jack that will probably be needed to be used many times and with a load of, possibly 1,000 lbs or more. Can anyone direct me to where I can find a scissor jack that would be capable of this and how I would go about finding one? Thanks, the posts have been very helpful.
Also, I am going on to get my Masters, possibly at Penn State, along with, possibly a Ph.D. Any advice on the value of Ph.D. and what it can be used for? Is it only valuable for academia or can it be equally useful in industry?
Thanks, I appreciate any feedback.

 

[GregLocock]

Define 'many times'. 100, 10000, 1000000?

As to the PhD, Check out the number of job adverts specifing a Masters. Divide by the number of MSc engineers graduating each year. Ditto with PhDs.

If you have a burning desire to investigate something to the nth degree then go for it, just don't expect too many other people in the real world to share your enthusiasm.



Cheers

Greg Locock

 

[dcme2]

Thanks for your input Greg. I'll take it into consideration.
any info on calculating air fuel ratio for four stroke, otto cycle, internal combustion, single cylinder, honda g200 engine?
Thanks,

Dan

 

[Fredbert]

Hi Dan.

1. Whats the fuel
2. How's the fuel put in.
3. Hows the air put in.

Basically work out your stoichiometric fuel/air requirement,
then add some air. Job done.

Cheers

Fredbert