Minimizing drag in a system of pulleys

[bobthebuilder]

Building a DIY kayak simulator training machine. A rope is tied to each end of a broomstick and runs around a resistance wheel, guided by a system of seven pulleys .

The machine works exactly as intended. But I feel there is too much drag. Trying to see how I can reduce the drag,in the system.

Transfer pulley is a 4" clothesline pulley

Remaining pulleys are 1 1/2" stainless block mounted pulleys.

Rope is 1/4" double braided marine rope.

Possibles -- replace stainless pulleys with nylon or ceramic? Replace woven rope with wire rope?


Thanks -- Bob

 

[DaveC426913]

• Larger diameter pulleys?
• Less "wrap" around the pulleys? (It looks like #1 right and left pulleys have about 250 degrees on them, any reason that can't be 70 degrees?)
• Are the pulleys fixed, or free to pivot? If you can fix them, even somewhat, that may help.
• Double-braided marine rope might have some inherent twist.

I am spitballing. I am not a mech eng (but I am a sailor, so familiar with friction in running lines ).

 

[bobthebuilder]

• Larger diameter pulleys?
• Less "wrap" around the pulleys? (It looks like #1 right and left pulleys have about 250 degrees on them, any reason that can't be 70 degrees?)
• Are the pulleys fixed, or free to pivot? If you can fix them, even somewhat, that may help.
• Double-braided marine rope might have some inherent twist.

I am spitballing. I am not a mech eng (but I am a sailor, so familiar with friction in running lines ).

Don’t be misled by the sketch; it’s a 3D system so 2D doesn’t do it justice.

All pulleys are apx 90 degrees except the transfer pulley is apx 180

 

[DaveC426913]

Are those lines run through raw holes in the lumber? You could be picking up a lot of friction there.

I do think a thinner cable would reduce friction. There are high performance lines that hold the same force as 1/4", but are much thinner. And it's an easy fix,

 

[bobthebuilder]

Yes, raw wood holes, but 1-1/4" diameter so there shouldn't be muchfriction there; just there to keep the rope from walking off the resistance wheel.

The only planned-in friction is the stainless fairlead that directs the rope to the intake pulley

 

[berkeman]

I'm not able to tell from the drawings -- does the resistance wheel change directions with left/right rowing, or does it keep rotating in one direction like a typical rowing machine?

 

[bobthebuilder]

Good question. Resistance wheel goes in the same direction both sides. Resistance wheel swiped from a Nordic Track ski machine; I had to reverse on of the rollers.

 

[Baluncore]

A kayak is pushed forwards by the paddler's foot, on the opposite side to the active paddle stroke. The power comes from body-trunk torque and glutes, not from the arms. The arms are more like struts or links, while one wrist only controls the blade angle. The body rocks backwards and forwards with each stroke, that also lengthens the stroke in the water.

Your feet and seat are co-linear, so do not prevent sideways rolling movement. A kayak paddler will have knees bent and apart, braced against the upper hull. You also need sides to the seat. That will balance side forces when paddling.

You will hurt your back with your arrangement because your waist-trunk will be locked, while your arm muscles will be doing the work. That is the opposite of paddling a kayak.

 

[bobthebuilder]

Thanks for the feedback. Well aware of the opposing foot-to-arm force in the paddle stroke. . . a buddy of mine had to cut back on his paddling cos he has a bad knee! ... as well as trunk rotation. Still fiddling with location of the foot rest . . . which is why it's secured with screws, not nails. As to the seat, a tractor seat on ebay was the best I could find.

FYI this is the device I'm using as a model

 

[jrmichler]

My kayak hoist has 3 pulleys and 3/8" twisted rope. The pulleys are hardware store pulleys about 2" diameter. Angles of wrap of the pulleys are approximately 180, 110, and 70 degrees. I attached a spring scale and it needed 85 lbs to raise the kayak, and 35 lbs to lower it. The difference is friction.

I think that your largest source of friction is the small diameter pulleys, along with the rope diameter. I suggest changing to pulleys at least 3", or better yet 4", diameter with your 1/4" rope. The pulley material is not important as long as the bearings are at least Oilite or plastic.

 

[bobthebuilder]

This surfski video shows how the paddler's arms and legs engage .

 

[bobthebuilder]

My kayak hoist has 3 pulleys and 3/8" twisted rope. The pulleys are hardware store pulleys about 2" diameter. Angles of wrap of the pulleys are approximately 180, 110, and 70 degrees. I attached a spring scale and it needed 85 lbs to raise the kayak, and 35 lbs to lower it. The difference is friction.

I think that your largest source of friction is the small diameter pulleys, along with the rope diameter. I suggest changing to pulleys at least 3", or better yet 4", diameter with your 1/4" rope. The pulley material is not important as long as the bearings are at least Oilite or plastic.

Good to know. Not aware what my bearings are made of...

 

[jack action]

The machine works exactly as intended. But I feel there is too much drag. Trying to see how I can reduce the drag,in the system.

I don't understand how friction causes too much drag when you add a resistance wheel. Can't you just reduce the resistance from the wheel?

 

[bobthebuilder]

I guess I want to be able to control the drag. That is, so the only source of rewsistance is from the resistance wheel

 

[DaveC426913]

In that case, those fairleads:

would be better as upright lead blocks, thus:

Never mind, youre just using them as straight guides. I thought they were going through a 90 degree turn, but they're not.

 

[DaveC426913]

While you're at it, you might want to replace that decaying old component between seat and paddle with a newer, hotter one. Lotta friction there!

 

[berkeman]

This surfski video shows how the paddler's arms and legs engage .

I'm not seeing any leg motion. How can they contribute much work if the lower body is not moving? It's not like a scull with a moving seat...

I'm not able to tell from the drawings -- does the resistance wheel change directions with left/right rowing, or does it keep rotating in one direction like a typical rowing machine?

Good question. Resistance wheel goes in the same direction both sides. Resistance wheel swiped from a Nordic Track ski machine; I had to reverse on of the rollers.

I'm not able to see from your drawings how the two rope inputs are able to drive the resistance wheel in the same direction. Do you use two bicycle freewheel gearsets to accomplish this?

 

[bobthebuilder]

In the video you can see the knees go up and down as the paddler drives his foot on the paddle stroke side.

The resistance wheel is from an old Nordic Track. As designed the rollers push the wheel on the forward slide and are idle on the back step. Reversing the roller on one side allows both paddle strikes to push the wheel in the same direction.

 

[Lnewqban]

Replace woven rope with wire rope?

I vote for that simple solution, besides reducing the diameter to the minimum acceptable for the forces at play.
Consider that a good portion of the drag force comes from simultaneously bending the rope at each pulley.

 

[berkeman]

How about even simpler? Two resistance wheels, and, you know...