Scissor lift help! 1

[stuart cox]

Hi all,
I'm hoping someone can help! I have built a Hydraulic scissor lift table for my motorcycles. I need to work out how much weight the hydraulic pump needs to be able to lift.
The table will be lifting up to around 500kg. The scissor arms are 875mm long from the scissor pivot. The hydraulic ram connected to the scissor arm 405mm from the scissor pivot point. The ram sits at an angle of 7° from floor level when the table is at its lowered point.
As the load on the pump will be at its strongest when the ram starts to lift the table, thats the calculation I need to work out how much weight the pump/ram will be trying to push at that point?
Any help would be much appreciated!
Thanks
Stuart

 

[3DDave]

I prefer the work method for a bottom-end estimate. Add some to account for friction in the joints.

To do this look at what happens to the linkage if the ram extends 1mm. If the table rises 2mm then you need twice the force in the ram (Force * distance) = work in = work out.

 

[stuart cox]

Thanks for the reply 3DDave.
When the ram extends 1mm the table lifts 5mm, so am I understanding this correctly please, 500kg weight on the table x 5 (5mm ÷ 1mm) = 2500kg. The hydraulic pump and ram need to be able to push 2500kg + the weight of the table top and a bit extra?
Many thanks
Stu

 

[TheTick]

stuart_cox your assumption is correct. Mechanical advantage is immutable, regardless of what is happening in the rest of the system.

One can use a parametric CAD sketch (e.g. SolidWorks) to calulate mechanical advantage of lift mechanisms. Apply a 1mm "wiggle" to the actuator and measure the change in height of the load. If 1mm of actuator movement yields 5mm of load movement, then your mechanical advantage is 1:5, therefore you need 24,525 N force to move 500kg (500kg * 9.81 m/s^2 = 4905 N).

We found at loads like these that joint friction is negligible if joints are properly designed. Add 10% for good measure.

What you also must determine is actuator power (force * speed). How fast will the actuator move at a given load?

There is a mathematical solution that can be determined by traditional statics. I'm on vacation and that math ain't free.

Stop using kg for weight. Kg is mass; N is force.

 

[stuart cox]

Thanks for the information and replying while on vacation 'The Tick' it is very much appreciated!
Many thanks
Stu