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Lever Arm Question

Lever Arm Question

Lever Arm Question

(OP)
I have attached a picture outlining a setup that I have a question about.

In this setup, there is a load that is being tilted around a pivot point. It's center of mass is located a bit off body, shown by a dot in the picture labeled CM.
Space being an issue a proposed solution was to extend a tab from the pivot point and pull from it using a linear actuator.

I would like to extend this problem such that if anyone can see any major downsides & issues with this setup that they might share them.
If anyone would like more information, it can be provided as requested.

RE: Lever Arm Question

While I don't see any real problem making this work, it's just that there will be a lot of rotational or Angular Momentum when the 'L-shaped' item swings up and stops after 110° of motion, particularly if it's moving fast.

John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Digital Factory
Cypress, CA
Siemens PLM:
UG/NX Museum:

To an Engineer, the glass is twice as big as it needs to be.

RE: Lever Arm Question

and don't forget that the linear actuator will also rotate about its pivot point. Not knowing how much rotation the linear actuator will undergo or the physical dimensions of the system, be careful about the dead end positions of the piston when it is fully extended or retracted.

RE: Lever Arm Question

and the linear actuator limits how much work can be done by the L as it rotates.

your 110deg arc is one limitation on the travel of the linear actuator, the radial distance from the pivot is another.

another day in paradise, or is paradise one day closer ?

RE: Lever Arm Question

Hi

I see no big issues with the set up however the linear actuator arm is smaller in the length from the pivot when compared to the length of the arm marked with cm, this means you have no mechanical advantage, if the actuator arm was longer than the latter then you could use a smaller actuator possibly.

RE: Lever Arm Question

(OP)
@JohnRBaker - Yeah. The momentum that it is going to be carrying was something that we were thinking about. The rise time is about 1.5 seconds. The load is gripped fairly tight but I am curious if there would
be extra strain on the actuator.

@chicopee - The tab off the main pivot is 4.8". So the 110° sweep it makes as well has an arc length of ~ 9.25". The stroke of the actuator is 8-10". So the radial movements are pretty straight still.

@desertfox - We are pretty pressed for space. I can see either pulling on the tab as shown in the picture or pushing the load up from the bottom as a simple solution. Could you elaborate further. To me having a larger actuator arm and having a smaller actuator are contradictory. Unless you mean the tab length.

@IRstuff - I would like to keep it a [Linear Motion] -> [Rotary motion] mechanism. Getting an extra rotatory motor in the current setup could be an issue.

RE: Lever Arm Question

(OP)
Would something like this have been better:
http://www.wordy.photos/index.php?keyword=dwell%20...
Or having an extra member between the end of the linear actuator rod and the tab?

Is there an advantage to either of those setups compared to allowing the linear actuator to pivot?

RE: Lever Arm Question

do you want a continuous cyclic motion, or open-hold-close ?

another day in paradise, or is paradise one day closer ?

RE: Lever Arm Question

(OP)
It wont be continuous. That "L feature' will be holding a load. It will move 110°. Dump it then return back to 0°. 1.5 seconds up. 1.5 seconds down.
The linear actuator starts extended with the tab at 215° (270 -55) and finishes pulling back at 325° (270 + 55)

RE: Lever Arm Question

is it worth talking about weights and lengths to appreciate sizing the actuator ?

another day in paradise, or is paradise one day closer ?

RE: Lever Arm Question

(OP)
Here are some better images explaining the mechanism.



RE: Lever Arm Question

(OP)
Member AB is 4.8"
From pivot A to the end of the "dumper" is ~ 21"

RE: Lever Arm Question

The mechanism looks quite a bit different now, however all I was meaning is that the actuator size could be reduced because it doesn't have to apply as much force if the length of the lever from the pivot to the connection point on the actuator is longer than the distance from the same pivot to the point marked CM, when I made my original comment there was no mention of limited space.

RE: Lever Arm Question

(OP)
Understood. If we end up going with the "Config 1" mechanism we will be extending slightly the AB member to help reduce the force. It is still considerable though. The actuator, I'm told, can handle the extra forces
fine.

RE: Lever Arm Question

  1. You need to look at the forces during the entire stroke.
  2. There may be some reversals from tension to compression due to the location of the c.g.
  3. If that is an air cylinder, the load might actually need to be restrained at some portions of the stroke.
  4. Flow control may not work if you are trying to get cycle speed, as you will need to flow control for the whole stroke.
  5. A hydraulic cylinder would provide more controllable movement.
  6. The original discussion of a linear actuator, meaning a machine screw actuator or a ball screw actuator may be a better choice if it can make cycle time.
  7. To me the ideal driver would be a rotary indexer which would have a nice acceleration and deceleration.

RE: Lever Arm Question

There is almost certainly a load reversal as the CM moves to the left of the pivot. Any backlash in the system will produce an undesirable acceleration during the reversal. Adding a counterweight near point B will help and also reduce the actuator load.

je suis charlie

RE: Lever Arm Question

(OP)
Ok, so when figuring out a counter weight. Am I correct, that when we have our final arm design and know our CM location, the Counter Weight will be placed at the opposite angle made my the CM?

As shown below.

Such that we get the CM as close to Pivot A as possible to make the motion easy to control.

RE: Lever Arm Question

Counterweight works okay but adds to the inertia of the system at start and stop, plus whatever you are moving into position is transferred out before the return stroke not sure how much your product weighs. You need to analyze some forces and then iterate the geometry.

RE: Lever Arm Question

Hi

Depending on how fast you want the mechanism to move and how much mass the linkages have compared to the forces the mechanism is seeing, you could keep a slight positive pressure on the annular side of he cylinder to act as a counter balance when the CM point goes over centre which would remove the requirement for adding physical masses to the system.

RE: Lever Arm Question

(OP)
We have changed the geometry of the arm a few times now, removing material where we can to reduce the overall weight. Now instead of pulling from the bottom lever, we have room to push from the right of the arm itself. It means a longer stroke actuator and pushing further from the pivot.

As for the product. It has negligible weight compared to the container that is moved holding it, and the arm holding the container. I really didn't like the forces I saw with the original system. However. there is more inertia with the Counter Weight and doubling the mass of the system to get the CM at the pivot might not be worth it. I am informed that we had air cushions helping before as the CM went over the 90°, so we had back pressure helping before.

So next question: If instead of using a large counter weight to pull the CM back to the pivot, I used a smaller CW facing down to pull the CM at 90° to the pivot, would that solve issues with the CM going over the pivot in the last 20° of the 110° arc? Such that at the top of the front cycle the CM would be directly vertical to the Pivot.

RE: Lever Arm Question

How many times are you going to perform this operation?

If the answer is a lot of times then look at a rotary indexer with rigid couplings - you are going to bang the heck out of that contraption slamming into cushions on cylinders.

http://www.camcoindex.com/ID_PARA.HTM

RE: Lever Arm Question

(OP)
18 cycles /min . 5 hours a day . 5 days a week.

I really do like the idea of a rotary indexer, there simply isn't the room for one. In addition cost becomes an issue too. The cylinder might cost us 80$ and be over designed by x8-12.
Rotary motion means gearbox, servo, wires.
I think if the project allowed to flip the product and have a next indexed set of arms ready for another, it would be great.

Still, this exercise has shown that a counter balance reduces the overall force on the cylinder and would be a good option for certain future projects.
It also allowed us to redesign the arm such that the Center of Mass did not move past the vertical.

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