Mechanical Advantage on Gym Machines

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Discussion Overview

The discussion revolves around understanding how a gym machine can reduce resistance by half throughout its range of motion, focusing on the mechanics involved, particularly the role of force angles and mechanical advantage. The scope includes theoretical and conceptual aspects of mechanical systems in gym equipment.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant, Alex, observes that the weight at the handles changes from 20kg to 10kg throughout the range of motion and suggests that this may be due to changes in force angles within the machine.
  • Alex proposes that the mechanical advantage increases as the angle at the handles approaches 90 degrees, but expresses uncertainty about this interpretation.
  • Another participant agrees that angles likely affect mechanical advantage and suggests using a Free Body Diagram to analyze the situation more precisely.
  • A different participant notes that the velocity ratio can vary significantly in mechanical systems, particularly in arrangements like pulleys and levers, which may not behave ideally across their range of motion.

Areas of Agreement / Disagreement

Participants generally agree that angles play a significant role in the mechanical advantage of the gym machine, but there is no consensus on the exact mechanics or implications of the observed resistance changes. The discussion remains open with multiple perspectives on the mechanics involved.

Contextual Notes

Limitations include the lack of visual aids due to lost images, which may hinder a complete understanding of the machine's mechanics. Additionally, assumptions about ideal behavior in mechanical systems are not fully explored.

Who May Find This Useful

This discussion may be useful for individuals interested in mechanical engineering, physics of machines, or those seeking to understand the mechanics of gym equipment.

reidos5000
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Hi All, need some help to work out how this gym machine is able to have the resistance reduce by half throughout a range of motion (from start to finish).

In the first picture (start of range), that 20kg plate on the end of the machine weights 20KG's at the handles (on the other end) when I weigh it with a scale.

In the second picture (end of range), that same 20kg weight only weighs 10KG's at the handle I'm pulling on.

My thought is it's due to the changes in force angles inside the machine. I can see that my mechanical advantage is increasing as I go through the range of motion where the angle at the handles I'm pulling on move closer to 90 degrees. I think that the machines mechanical advantage is reducing due to the changes in those force angles inside the machine moving away from 90 degrees right? But I'm not sure I have this right.

Can anyone confirm if I'm on the right track here? Are there aspects I'm missing that make this resistance decrease significantly?

Any insights would be greatly appreciated.

View attachment 364317View attachment 364318Thanks
Alex

[Mentor Note: Two images of a Universal Weight Machine being used for Military Press have been lost. A similar image is pasted in below for reference.]
1755007398510.webp
 
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Yes, the angles make a difference and are most likely the source of the mechanical advantage changes.

Do a Free Body Diagram to determine exactly how.

Free-body-diagram-of-the-four-bar-mechanism-that-joins-the-actuator-and-the-base-link-of.jpg
 
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Great, thanks!!
 
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reidos5000 said:
how this gym machine is able to have the resistance reduce by half throughout a range of motion
The same velocity ratio (i.e. in the absence of friction and dead weight!!) can only be achieved with pulleys and single pivoted bars. Other arrangements can be expected not behave ideally.

Even a simple L shaped lever will produce a varying velocity ratio over a range of angles. A pulley with a crank (like a bicycle) will vary a lot over 180 degrees of movement.
reidos5000 said:
[Mentor Note: Two images of a Universal Weight Machine being used for Military Press have been lost. A similar image is pasted in below for reference
Thanks for the image but, not being a gym user, I can't see what's what. Perhaps the OP could produce another link (or just a copied image) so we all know what's going on.
 

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