Trying to determine formula for a certain type of lever

[Fysicsisphun]

So I'm trying to figure out how to calculate the force behind the pressures achieved in this press. I know the pressures achieved with specific amounts of water(weight) for the pivot point being at either pin a or b but I'd like to know how to calculate this myself. I've tried looking up different types(orders) of levers but I can't find a formula that seems appropriate. It's been a while since I've taken a physics course

 

[Simon Bridge]

Welcome to PF;
It's a class 2 lever
https://en.m.wikipedia.org/wiki/Lever

 

[sophiecentaur]

What does that press actually do? I am intrigued. Is it just a nicely made demonstration of pressure and the Principle of Moments?
If you want to calculate the pressure as accurately as possible then the details of the actual machine are also relevant.
Look at this hyperphysics link for an alternative view of things. You will notice that, on the link, the "Mechanical Advantage" is referred to as "Ideal Mechanical Advantage", which is a warning that the MA that people calculate is often way out because of the dead weight of parts of the mechanism and due to friction forces. When I was at school, IMA was referred to as Velocity Ratio (VR).

 

[Fysicsisphun]

Hi Sophiecentaur,

The press is used in the cheesemaking. You load the end of the arm with a weight and it presses down onto the cheese which is near the fulcrum exerting a force which presses out the whey.

And thanks Simon, that's what I was thinking too but I was confused since in a class 2 lever the force exerted is going up against gravity while in this press the force is directed down. Would you just treat that as a negative number then?

I guess I should have included more details as to what this thing was.

 

[sophiecentaur]

Ahh. Cheese making.
You are talking in terms of "up" and "down" for the lever problem and you have a problem with the signs. Because levers can have many forces applied to them and in all directions, it is usual to talk in terms of Clockwise and Anticlockwise Moments, rather than up and down. So the milk bottle is providing a Clockwise moment about the pivot (A or B) and the reaction force from the cheese piston is providing an Anticlockwise moment. There is an additional weight force on the cheese in your press because of the inherent weights of the arm and the vertical rod (the 3.6lbs). In your example, the force is magnified by the ratio of the distances from the pivot.

The basic principle of moments is very straightforward. Most elementary treatments just show a 'see saw' example but the majority of levers that we come across will be crank shaped, where an up and down movement produces a side to side motion. The same Moment and torque methods are also applied to wheels, gears and pulleys. The approach is to choose a pivot (fulcrum) and add all the clockwise (Force times perpendicular distance to pivot) moments about the fulcrum and the anticlockwise moments. There will be an 'unknown' in the equation, which you 'solve for. Some basic algebra which can be solved as long as there is only one unknown.

 

[Fysicsisphun]

Hi Sophie centaur,

Thanks for your explanation, you're taking me back to my college physics days:) I'm going to try and back calculate the forces in this press so that I can design my own press with slightly different pressing forces.

Thanks again for your help, and I expect to ask further questions when I get into designing and building my own press.

 

[sophiecentaur]

Hi Sophie centaur,

Thanks for your explanation, you're taking me back to my college physics days:) I'm going to try and back calculate the forces in this press so that I can design my own press with slightly different pressing forces.

Thanks again for your help, and I expect to ask further questions when I get into designing and building my own press.

OK. It is very simple really. It's all to do with the ratio of the lengths. You can even do your own experiments with bathroom scales instead of the cheese cylinder. Have a go.