Mechanical Advantage on Gym Machines

[reidos5000]

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.]

 

[jack action]

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.

 

[reidos5000]

Great, thanks!!

 

[sophiecentaur]

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.

[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.