Mechanical Advantage of a pulley with horizontal effort

[Fiona Rozario]

How will the MA of a fixed pulley change if I apply the effort horizontally? Intuitively, I feel horizontal effort should be lesser effort than vertical effort.

 

[BvU]

Same force, other direction -- why would that change the mechanical advantage ?

 

[Lnewqban]

Summary:: How will the MA of a fixed pulley change if I apply the effort horizontally? Intuitively, I feel horizontal effort should be lesser effort than vertical effort.

Could you show us a schematic related to your question?
In general, fixed pulleys do not provide any mechanical advantage, only change the direction of the applied tension.

 

[Fiona Rozario]

Sorry, I should have asked my question the other way round - Why doesn't the MA of a pulley depend on the direction of the effort?

 

[Lnewqban]

Sorry, I should have asked my question the other way round - Why doesn't the MA of a pulley depend on the direction of the effort?

Please, don't be sorry.
The MA of a pulley depends on the angle of contact of the rope.
Please, see:
https://www.physicsforums.com/threa...ley-vs-an-anchored-pulley.985050/post-6306703

 

[Fiona Rozario]

Thank you, so much!

 

[Lnewqban]

You are welcome, Fiona

 

[BvU]

Please, don't be sorry.
The MA of a pulley depends on the angle of contact of the rope.
Please, see:
https://www.physicsforums.com/threa...ley-vs-an-anchored-pulley.985050/post-6306703

That is not what is meant with the mechanical advantage of a pulley! The picture shows the relationship between the magnitude of the two forces drawn in red and the force in black. Mechanical advantage is a result of the number of loops in the rope (minus friction).

 

[Lnewqban]

That is not what is meant with the mechanical advantage of a pulley! The picture shows the relationship between the magnitude of the two forces drawn in red and the force in black. Mechanical advantage is a result of the number of loops in the rope (minus friction).

That is absolutely correct, the higher the number of loops in the rope, the higher the MA of a system of pulleys.

Perhaps I have not understood the OP's question (later modified by post #4) and have given incorrect advice.
My apologies to the OP if that is the case.
Again, posting a schematic diagram in the OP is always helpful.

What I have tried to explain above is that the mechanical advantage of a pulley is less than the ideal value of 2 each time that both sections of the loop are not parallel to each other.

If we accept the definition of IMA provided by the following article, the ideal ratio of the force out of the machine (load) to the force into the machine (pulling effort) is reduced by any lack of parallelism of the loop for a single pulley.

https://en.wikipedia.org/wiki/Mechanical_advantage#Ideal_mechanical_advantage

 

[sophiecentaur]

That is absolutely correct, the higher the number of loops in the rope, the higher the MA of a system of pulleys.

This is not necessarily true in practice because it depends on the weight of the pulleys. This is why I always recommend using the quantity Velocity Ratio which is ideal and depends only on the geometry. As soon as you get practical, MA is less than VR (sometimes vastly less). Pulley blocks are often actually heavier than the load and significant work has to be done to lift them. (Not to mention the friction throughout)
I do not understand the reluctance to use the correct term. Two different concepts require two different terms.

 

[Lnewqban]

... I do not understand the reluctance to use the correct term. Two different concepts require two different terms.

Would you mind elaborating about the correct term a little more?
I am sorry, sophiecentaur, but I don't follow your reasoning here and feel I am missing something important.
What terms would you use for each case?

The way I see it is that, velocity ratio is only applicable under the ideal assumption that power-out equals power-in, since power = force x velocity, as you well know.

I believe that the power that is consumed by the stretching, bending and skidding of several loops of rope, the bearing's friction, the dead weight of pulleys, etc. is considered in the force-out / force-in ratio of what I call actual mechanical advantage of a simple machine with less that ideal mechanical efficiency.

If I may use the example of a simple lever, the difference of velocities between the input and load ends will be about the same for ideal and actual conditions.
At the same time, for lifting the same load, the actual less efficient lever would require a greater input force.

 

[sophiecentaur]

OK. Take your lever as an example. The VR is the geometry of the lengths. Assume you are using a galvanized scaffold pole to lift a walnut with the pole jammed into the ground. The VR will be say 100 but you are Also lifting that pole - half its weight- and 1/100th of the weight of the nut. MA is Approximately 1/100 of the weight of the nut divided by half the pole weight. Common use of MA would not consider that so go get the pole and be prepared for a surprise.
Some very efficient machines do a lot better than that, of course but you cannot assume 90+% efficiency.

 

[Lnewqban]

...
Some very efficient machines do a lot better than that, of course but you cannot assume 90+% efficiency.

Now I get it.
Thank you.

 

[sophiecentaur]

Now I get it.
Thank you.

Problem is that, if you take my message and start to use VR, most people will say "Whaaaat?" and think you are speaking Greek.

 

[Some random guy]

You all speak Greek lol. I just wanted to tell everyone of you y'all should be extremely proud of yourselves. All of you are extremely talented individuals, and also the professionalism and professional courtesy towards one another is outstanding. And these are just some appreciative and encouraging comments from somebody from nowhere LOL

 

[sophiecentaur]

You all speak Greek lol. I just wanted to tell everyone of you y'all should be extremely proud of yourselves. All of you are extremely talented individuals, and also the professionalism and professional courtesy towards one another is outstanding. And these are just some appreciative and encouraging comments from somebody from nowhere LOL

The message in all this is that 'Machines' are used, either to produce more force / torque or more movement. Mechanical Advantage is essential a practical quantity and it's 'what you get', after the geometry, friction and deadweight are all taken into account.
The General Public are sloppy about many things and they will expect things like 'leverage' always to be in their favour - when they just look at the layout. Anyone with Engineering aspirations needs to learn the realities of these things.