# Scissors Lift Math?

• posted
My neighbor wants to build a scissors-type motorcycle
lift (as seen on the motorcycle-building shows) and has
asked me to help him design and fabricate it.
I'm thinking that the simplest and safest way to go
is to use a screw to provide the lift rather than
a pneumatic or hydraulic cylinder. I see that Enco
is selling 1" - 5 TPI ACME threaded rod and nuts,
which sounds sturdy enough for a ~1000 lb. load.
I'd like to use a reversible motor to crank the
screw.
The question is, how do I, even roughly, calculate
the minimum "comfortable" horse power required to
operate this lift? I'm figuring on using a single
scissors with the hinge point at mid-point on the
arms, so the ratio will be 1:1. I guess the missing
variable is the length of the arms...?
A related question has to do with gear reduction.
Will a steering box from an automobile work as a
gear reduction unit once the steering arm is removed?
Are they designed so that the output shaft will
rotate multiple times, or are they limited to only
a partial revolution?
Thanks
• posted
||A related question has to do with gear reduction. ||Will a steering box from an automobile work as a ||gear reduction unit once the steering arm is removed? ||Are they designed so that the output shaft will ||rotate multiple times, or are they limited to only ||a partial revolution?
Partial rev only, won't work. What might work is a differential from a riding lawnmower. You might also look into adapting a trailer tongue jack.
Your friend does know you can buy those hydraulic MC lifts for \$60, right? Texas Parts Guy
• posted
Curses, foiled again.
Hmmm, very interesting. Lawn mower trannys are usually pulley driven, too. Sounds easy to interface to a motor.
There are different types of MC lifts. The guy is thinking of a full length drive-on type. Were you thinking of the cradle type that rests the bike on its frame?
Thanks for the idea.
• posted
"Artemia Salina" wrote: (clip)There are different types of MC lifts. The guy is thinking of a full length drive-on type. Were you thinking of the cradle type that rests the bike on its frame? (clip) ^^^^^^^^^^^^^^^^^^^ I have one of those from Harbor Freight, and I find it very handy for various purposes. It uses a hydraulic jack. I like having it independent of outside power. Seems to me that it would be easier to make a roll on ramp and track for it than building something from scratch.
• posted
|| ||> Your friend does know you can buy those hydraulic MC lifts for \$60, right? ||> Texas Parts Guy || ||There are different types of MC lifts. The guy is thinking of a full length ||drive-on type. Were you thinking of the cradle type that rests the bike on its ||frame?
Yes, that's what I was thinking of. But the mechanism and capacity is already there. Why not look at one with an eye to adding a full-length bed? Outriggers to stabilize the base, and a bed extension. Fold-down legs at full height. Another possibility: Look at the low-cost scissor-lifts that some tire shops use. You just want a narrow version of that. Unless it's the building process that appeals to you. Texas Parts Guy
• posted
I actually home built a motorcycle lift like you describe not too long ago. I used a "3 ton long ram jack" from harbor freight that cost about \$40 to raise and lower. I found that they will still pump if fully horizontal as long as the little pumping mechanism is kept on bottom. The only downside to this approach is that to raise the stand up to the 34 inch working height took an awful lot of pumps - but I only use it for maintenance of my own bikes, not in a shop capacity so it didn't bother me. Also the lift had some catches that progressively engaged as it was being raised so if the hydraulics did fail it wouldn't come crashing down (I would actually release hydraulic pressure so it would sit on the catches when I was working on the bike). I have some picutres at home that I can post a little later. Also keep in mind that I never did any formal analysis on the structure of the thing and all my bikes are probably less than 450lbs so if you have a big 1000lb beast of a bike take that into consideration.
-Kris
Artemia Sal> My neighbor wants to build a scissors-type motorcycle
• posted
Power = force x speed. For rough numbers, say the weight of the bike and platform is 1200 lbs and you want it to lift at a rate of 1 inch/sec. (1 hp = 550 ft-lb/sec)
power = (1200 lbs)(1/12ft/sec) = 100 ft-lb/sec = 0.1818 Hp (say a 1/4HP motor)
The more important issue is torque. With a 5 TPI thread, the turning speed needs to be 5 rev/sec (300 rpm) to get a lift rate of 1 in/sec (ignoring the mechanical advantage of the scissor mechanism for the moment). Torque = power/rotation speed (in radians/sec).
Torque = (100 ft-lb/sec)/(10PI rad/sec) = 3.18 ft-lbs
Again, roughly, 1750 rpm motor/(300 rpm) = 5.83 reduction ratio.
Motor torque = 3.18 / 5.83 = 0.545 ft-lbs.
Allowing for friction losses, it looks like you're looking for a 1/4 or 1/3 hp motor with a 6:1 reducer on it.
I hope this helps Bruce
• posted
In penna all lifting device need inspeced
• posted
In article , Kris M. writes
Strewth! If it cost me \$40 to raise and lower a lift I'd sell if and get one that's cheaper to run...
• posted
What you need is an old trash compactor. They work on the scissor lift principle, have a reversible motor, fairly large range of motion, and all the necessary controls. Trash compactors are out of style right now, and most people want them out of their kitchen.

• posted
Thanks Bruce, it certainly does.
Thanks to everyone else as well.
• posted
i finally got a chance to post some pics. it not exactly a 'scisors lift' but it works well. the pics are at:
The hinged ramp attached to the lift was a bad idea. it's since been removed. I also built a wheel clamp to hold a bike by the front or rear wheel and added some eye bolts for tie-down straps to keep the bike really securely attached if needed. enjoy.
-Kris
• posted
NICE JOB!!!
My parent's shop used to have a bunch of HandyLift air benches and they were great to work on. Most import bike benches are flimsy and I wouldn't trust a Ducati on it let alone a dresser Harley or Goldwing.
There were three types of benches, one that used a double bellows airbag off a semi trailer for lift. It was fast and strong until someone left the airline on and blew the bag. (150+psi air line on a dry rotted bag.)
Another type was the air cylinder type. Fast and no part to blow out except for a single seal. Never went bad after years of daily service.
Our one Italian air over hydraulic bench was slow, poorly made, flimsy and blew oil all over the place. (Kinda like a FIAT!!) That bench got parked!
Your design looks plenty strong, but maybe a air bag or air cylinder (NOT hydraulic!) so it will go up and down faster.
Bart D. Hull snipped-for-privacy@inficad.com Tempe, Arizona
Check
for my Subaru Engine Conversion Check
Tango II I'm building.
Remove -nospam to reply via email.
Kris wrote:
• posted
Very nice!
Say...how come you didnt mill it out of billet?
Gunner
"There is no difference between communism and socialism, except in the means of achieving the same ultimate end: communism proposes to enslave men by force, socialism - by vote. It is merely the difference between murder and suicide." - Ayn Rand, from "Foreign Policy Drains U.S. of Main Weapons"
• posted
On Mon, 09 Aug 2004 11:17:11 GMT, Gunner calmly ranted:
Smart, Kris. But a >90° angle on the ramp with a roller on the bottom would have made it workable.
Yeah, nice.
That'd look even better with an expanded metal top vs. the ply, huh? And all chromed. And air-over-hydraulic automatic lifting/lowering. And... ;)
Great Googly Moogly, Gunner! Wazzat a Searz billet, or a real billet?
- Woodworkers of the world, Repent! Repeat after me: "Forgive Me Father, For I Have Stained and Polyed." -
Comprehensive Website Design
• posted
On 5 Aug 2004 16:53:44 -0700, snipped-for-privacy@thinkerf.com (Bruce Lehmann) vaguely proposed a theory ......and in reply I say!:
PMFJI. I tried to deal with this with a guy who was building one using a jack and having trouble.
The problem with scissor lifts, as I see it, is that when they first start and the scissors are "flat" the forces required to lft the thing are huge, because of the angles involved, unless you are very careful to get the angle of the actuating jack (screw etc) right. And that is difficult, because the whole thing has to be very flat when it starts, by nature.
As with hydraulics, you need to have immense forces, often with very poor mechanical advantages. The available force "at the workface" alters enormously with the actual position of the implement.
Someone suggested a garbage compactor. the thing is with a compactor. when the scissors are flat, the garbage is loose. As the scissors open, the forces get more efficient, and the garbage starts to get harder to compact.
The motor cycle stays the same. Without care, you could end up pushing forces 5-10X the weight of the bike. The initial lift will be fast but weak, and later lift slower but more powerful.
The trick, as far as I can see, is to make the angles such that you approach a situation when the angles move around, and stay near, some constant.
I actually fooled around with the idea of pulling (pushing?) on the intersection of two circular tracks. This can be set up to make the lift per actuator ratio pretty constant. Theory only.
***************************************************** It's not the milk and honey we hate. It's having it rammed down our throats.
• posted
This is a good post, Nick. This is one of the problems with scissors lifts, and I did some research on this when I built a few adjustable hydraulic carts for a stamping plant.
Jim Kovar Vulcan, Mi
• posted
On Mon, 09 Aug 2004 18:04:16 -0500, Jim Kovar vaguely proposed a theory ......and in reply I say!: