Lever Arm Question

[ApexM0Eng]

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.

 

[JohnRBaker]

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.
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[chicopee]

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.

 

[rb1957]

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 ?

 

[desertfox]

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.

 

[IRstuff]

Have you considered using a Geneva mechanism:

http://www.wordy.photos/index.php?p...ategory=tech&title=geneva+mechanism+3&video=1

TTFN
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[ApexM0Eng]

@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.

 

[ApexM0Eng]

Would something like this have been better:

http://www.wordy.photos/index.php?k...0&category=people&title=scotch+yoke+mechanism

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?

 

[rb1957]

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

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

 

[ApexM0Eng]

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)

 

[rb1957]

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

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

 

[ApexM0Eng]

Here are some better images explaining the mechanism.

 

[ApexM0Eng]

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

 

[desertfox]

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.

 

[ApexM0Eng]

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.

 

[dvd]

[ol 1]
[li]You need to look at the forces during the entire stroke.[/li]
[li]There may be some reversals from tension to compression due to the location of the c.g.[/li]
[li]If that is an air cylinder, the load might actually need to be restrained at some portions of the stroke.[/li]
[li]Flow control may not work if you are trying to get cycle speed, as you will need to flow control for the whole stroke.[/li]
[li]A hydraulic cylinder would provide more controllable movement.[/li]
[li]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.[/li]
[li]To me the ideal driver would be a rotary indexer which would have a nice acceleration and deceleration.[/li]
[/ol]

 

[gruntguru]

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

 

[ApexM0Eng]

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.

 

[dvd]

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.

 

[desertfox]

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.