How to Improve a Swing Arm system

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How to Improve a Swing Arm system

Postby Midniteblue » Mon Nov 12, 2012 1:31 am

I am rebuilding an Old Hickman EC-12 and as Sails Control I will use a Swing Arm System with a different approach.

I will use a Hitecc HS-5805MG servo, rated 343 oz. and speed .14 sec. and a Swing Arm not longer than 5 inches so I can secure the servo just on the lead.
Using Single Blocks w/Becket attached on the "Hull" and Double Blocks on the Swing Arm, I will be able to improve the Power of the servo to 1372 oz. at the same .14 sec. speed and have enough Main and Jib sheeting.

This can be accomplished because the mechanical advantage of the Pulley Physics that as described is equal 4 .

I Hope it will help somebody, the servo is very affordable and the all system is not complicated.

Val
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Re: How to Improve a Swing Arm system

Postby s vernon » Mon Nov 12, 2012 3:11 pm

I defer to Marc.
Last edited by s vernon on Mon Nov 12, 2012 6:36 pm, edited 1 time in total.
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Re: How to Improve a Swing Arm system

Postby marcsmith » Mon Nov 12, 2012 3:54 pm

Val,

Not sure you math is correct...

the torque is measure in ounces inch T= r x F r-radius f- force perpendicular to radius

so for your servo the torque is 343 oz-in so at one inch from the drive your servo exerts 343 ounces of force.

if you move out to five inches you divide 343/5 with means you servo now exerts only 68.6 ounces at 5". so for ten inches of sheet travel, assuming 180* rotation. your get 68.6 ounces of force and 10 inches of travel.

if you go with a double purchase system you are doubling the distance your servo can pull, 20 inches, but your halving the amount of pull to 34.3.

with the system you are talking about, @ 5" arm length you will get 272ounces of force 4x68.6 , but you will only get 2.5" of travel...10/4... or you can get 40 inches of travel but only 17.1 ounces of force

In other words, there is no free lunch. you can only get one mechanical advantage at a time... you either get the distance or the force, but not both.
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Re: How to Improve a Swing Arm system

Postby Midniteblue » Mon Nov 12, 2012 7:58 pm

I built a 2 meters Trimaran and I am using S. Barbara sails. Because the body of the Trimaran is narrow to have enough sheeting for the sails I used a system as described above. In the Trimaran I have an Ozmun winch that is rated about 180 oz.in The Trimaran works fine, but I never sailed her in strong wind.

Some more details; the total arm is about 5 inches, 2.5" on the Main side and 2.5" on the Jib side. On the arms I mounted the Double Blocks and somewhere in the Front and Back of the Hull the Single Blocks w/Becket . The sheet is secured on the Becket of the Single Block, goes to the 1st Pulley of the Double Block, return to the Pulley of the Single Block, back and around the 2nd Pulley of the Double Block and out to the sails.

Now, if as a person, my personal limit is to lift 100 Lbs using 1 Pulley but I can lift 200 Lbs using 2 pulleys, all of a sudden in a certain way I became twice as strong at the only expense of doubling the rope that I have to pull........????
Now apply this concept to the Arm Swing System. About the multiplier or mechanical advantage you can learn more following this link:
http://www.constructionknowledge.net/pu ... 201994.pdf

Guys, at this point for sure I know that I have control of the sails in the Trimaran and I will find out about the power of the Servo when I will test the Old EC-12 Hickman that I am working.
I am very confident that it will be a success independently of how to prove the numbers.

Val
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Re: How to Improve a Swing Arm system

Postby marcsmith » Mon Nov 12, 2012 8:57 pm

a fixed pulley on one end with a single rope, is a 1:1 you gain no mechanical advantage only the ability to change the direction of the force you are applying.


with the line fixed at one end, the load on a pulley, and the line going through another block, http://images.yourdictionary.com/block-and-tackle IE simple block and tackle you end up with a 2:1 mechanical advantage, ie you have sacrificed distance for the mechanical advantage. to move 200 lbs one foot, you must move 2 feet of rope. this is how you suddenly became "stronger"

this is essentially a double purchase system on a sail boat... the servo arm is your load, sheet starts at the boom through the fairlead, through the winch arm, and then deadmans in the hull in the direction of the fairlead

http://www.theamya.org/hints/images/mcivicfig03.jpg this setup doubles your sheet travel, but increased your load. essentially a block and tackle only working in reverse...

each time the line passes through a block it shares the load. int he RC world for those who want to stay with the arm setup for simplicity, we sacrifice load for distance. Very easy to get a more powerful servo and wire it to rotate 180 and throw in a double purchase. to get the needed sheet distance.

I'm not doubting that you get your distance, i'm doubting your math on the quadrupling of the load your servo can now take..

I drew up a picture of your setup...does it look like this...
Attachments
sheet.jpg
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Re: How to Improve a Swing Arm system

Postby Midniteblue » Mon Nov 12, 2012 9:04 pm

Yes it does. Thank you
Val
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Re: How to Improve a Swing Arm system

Postby marcsmith » Mon Nov 12, 2012 10:12 pm

ok, great...

if you look at the link you provided....

on page 2.3 your system looks like the luff tackle in figure 2.7

the difference between your setup and 2.7 is the end where the load is(the box) is fixed in your boat, and the double block is moving in your boat(servo arm) as opposed to being fixed in figure.

let say the load in the diagram is 600lbs. with three lines supporting each line supports 200lbs. it takes 200lbs of pull to raise the 600lbs. the mechanical advantage you have gained the fact the to move one foot, you need to pull 3 feet of rope.

so lets reverse this. put your boom on the end of the rope you would normally pull and lets fire up the servo.

in a one to one IE one pulley to only change direction, with the sheet secured to the end of the servo and the boom. for every inch of travel of the servo, you get one inch of travel on the sheet and zero mechanical advantage

with a double purchase. one end dead-manned in the hull, passing through the servo arm and then back through the fairlead. for every inch of travel of the servo, you are getting two inches of sheet travel 2:1. With a double purchase you are sacrificing the mechanical advantage for the travel advantage.

you have replicated a triple purchase with your setup. so for every inch of servo travel, you get 3 inches of sheet travel. again sacrificing the load mechanical advantage for the travel advantage.

you have to keep in mind with a luff tackle the load end(single pulley with becket) as pictured in the figure 2.7 the load moves as you pull the line and the double block is fixed.. so you pull 100lbs and 3 feet to get 300lbs to move one foot. with the sail servo, the load end is fixed, and the double block moves. so when the 300lb double block moves one foot, it pulls 100 lbs 3 feet...

remember the 3:1 mechanical advantage you get by pulling 100 pounds 3 feet to get 300lbs to move 1 foot work sin revers when the double block moves. it needs to exert 300lbs over one foot to move 100lbs 3 feet.

and I'm not even taking into account frictional losses, or the fact they you aren't really getting 343ounces of force 2.5" away from the drive of the servo...
taking the 2.5" distance you are only getting 137.2 inches of force tangent to the 2.5" radius so for 2.5" radius you have 5" diameter. which means with the setup you have, you are getting 15" of travel, but only 47.3 ounces of force at the boom. which is still a significant 2.85lbs.
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Re: How to Improve a Swing Arm system

Postby Midniteblue » Mon Nov 12, 2012 11:26 pm

Marc, thank you for the explanation.

Since we are in the subject, what if we swap the Blocks, we locate the Single Block w/Becket in the Swing Arm and we fix the Double Block on the boat and the sheet goes to the fairlead from the Double Block on the boat. Do we gain, lose, or nothing change.

Thank you for your help
Val
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Re: How to Improve a Swing Arm system

Postby marcsmith » Tue Nov 13, 2012 12:30 am

so you want to set it up like figure 2.7 from you link. IE set it up just like the luff tackle.

your servo now becomes the load. the double is dead maned and the sheet is still the "pulling" end. its still a 3:1 boom moves 3" servo moves 1" servo exerts 300ounces, the booms only see's 100 ounces, you really have not changed anything....

the only way to really change anything. would be to put the double block on the deck, the block/becket on the boom and the deadman the sheet to the servo. this will give you 3 times the pulling force. so your 300oz servo will now exert 900 ounces., but the problem is you need the servo to move 3" for the boom to move 1". this gives you a typical main sheet setup like you would find on a big boat... for a multi turn drum winch were sheet travel is not the issue but pulling force is, this is a setup you could use to increase your power at the expense of travel... but this generally is not the problem with model boats... we need sheet travel, and we need power... so on an arm winch we get a more powerful servo and put the biggest arm we can squeeze int he hull and then do a double purchase. effectively halving the power but doubling the travel.

in my ec12 i have the 280EL with a spiral drum. I get about 16" of travel with out any double purchases. the drum is about 1" on the outer rim so I'm getting about 260 ounces of pull...real 260. and with the spiral wind, as it get closer to sheet-in the power is effectively increasing(call it a lower gear) while i sacrifice my travel. which is good in that I get a finer adjustment on sheet-in....

hope I have not completely confused you...or taken the wind out of your sails.... :wink:
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Re: How to Improve a Swing Arm system

Postby Midniteblue » Tue Nov 13, 2012 2:02 pm

Marc, thank you again for all the explanations, it has been an excellent mental exercise for myself; I will definitely be able to be analytical in this subject.

Val
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