Pulley tension question

[dpsmith85]

I'm trying to find bearing pressure on a pulley so that I can select the proper one for the design. I know that the bearing pressure will be the (line tension x angle factor)/(shaft diameter x hub width). In our case the angle factor is 2 because the angle between the lines coming off of the pulley have are close to 0 degrees. There is a picture attached which shows a free body diagram of this in the right hand corner. Our line tension is 2000lbs therefore the force on the pulley is 4000lbs. That makes perfect sense to me if there is only one pulley. Also in the attached picture shows our current system which uses 7 pulleys and a winch to lift 2000lbs. In my head I want to take the 2000lbs and divide by 7 and get that each pulley needs to hold 285lbs, but I know that's got to be wrong. I remember from school that the line tension stays the same over each pulley it simply changes direction. With that in mind it would mean that each pulley would have the same free body diagram like the one in the right corner and each pulley needs to support 4000lbs. Is that really the case? Do each of these pulleys really need to support that much load? Can someone help explain why this is? I just don't want to over/under design this.

-Thanks

 

[btrueblood]

Your diagram shows one rope attached to the load, and no pulleys attached to the load. So, the load on the rope is 2000 lbs, and the load on the first pulley is approx. 4000 lbs.

The only way to achieve smaller line loads is to use a block and tackle arrangement, i.e. multiple ropes attached to load.

http://www.google.com/url?source=im...AUQ8wc&usg=AFQjCNGkQkcLjNcECXfZ8Bti3BjQu_fMZw

 

[dpsmith85]

Thanks for the reply. I do need to clarify that I'm not concerned about any mechanical advantage. Our current system doesn't need it. Each pulley is only used to change direction. My question is really based on how much load is each pulley having to bear? Like I said it makes sense that if there was just one pulley it would need to bear 4000lbs but since there are multiple pulleys on the same line it confuses me. Does each pulley need to bear the same 4000lbs? Or can it be considered that the 2000lbs load is "spread" across 7 pulleys?

 

[leisure17]

4000 on each pulley

 

[dvd]

If one were lowering a load with a pulley system that had several pulleys, the friction at each pulley axle would create a moment that would cancel some of the moment created by the lowering force. If trying to raise the same load, with the same system, the frictional forces and the force to lift the load would be additive because the moment at each axle would be in the same direction. So for lowering, it may be possible to have a tension less than the load; but for raising, the tension would be more than the load.

(The forces to accelerate the load from rest are not included.)

 

[chicopee]

dvd has the correct response. See my reply to thread 406-306045 interestingly enough has a somewhat similar question.