Capstan project

You've omitted some key parameters:

capstan speed when motor is running at rated speed and power breaking strength of rope

Don, everyone

My calculations in the raw - my notes of the time. Hope you can read the Lisp syntax - eg. (- 5 2) gives ans. 3

Here goes...

; 15 April 2010

; Want to make capstan

; Found by observation of turning the input shaft that the reduction ; ratio is 50:1

; ----------------------------------------------------------------

; Taking idea of motor like Briggs and Stratton - reckon revs ; something like 1500RPM ; Others think something like 3000RPM ; Both these are the maximum flat-out RPM estimates ; Look up Briggs and Stratton engine.

(/ 1500 50) ; 30 (/ 3000 50) ; 60

; these are the shaft output RPM's - just right for a capstan.

; --------------------------------

(setf *WARN-ON-FLOATING-POINT-CONTAGION* NIL)

; How rapidly do we want the capstan to inhaul? ; What can we say about desired characteristics?

; Well, ability to pull at 1.5Tonnes-f tension would go to near the ; maximum of a 12mm dia rope.

; Attach a snatch-block to the load and route the rope back to an ; anchor by the capstan and you are up to 3Tonnes-f - useful!

; From this, can calculate inhaul rate given power of motor - ; work-done = force x distance ; power = force x velocity

; Say output power is 3HP (about 5HP at the motor)

(defconstant +W-per-hp+ 748 "the number of Watts per HP as defined") (defconstant +g+ 9.81 "Gravity - N kg^-1 or m s^-2")

(setf power (* 3 +W-per-hp+) ; 2244W draw-force (* 1500 +g+)) ; 14715N

(/ power draw-force) ; 0.15249744 m s^-1

(* 0.15249744 60) ; so that's just under 10m per minute (9.14m)

; where this leads to is what diameter the capstan drum has to be

; If capstan drum were 100mm diameter

;; 19 May 2010 ;; circumference ;; 100mm = 0.1m ;; C = Pi x D ;; (* 0.1 pi) 0.3141592653589793m ;; but the capstand is turning at 0.5rpm so:

(* 0.5 (* 100e-3 pi)) ; 0.15707964 m s^-1

;; 19 May 2010 - I plucked 100mm out of thin air and it proved a ;; spot-on guess - at 1500rpm the motor has the power to inhaul just below ;; the breaking strength of the rope at 0.15249744 m s^-1 and at ;; 1500rpm with a 100mm dia capstan (at the waist) the rope would be ;; inhauled at 0.15707964 m s^-1 ie. 0.15 m s^-1= 0.15 m s^-1

; So what this says is - if max. power of 3HP were developed at ; 1500RPM, then all this fits together perfectly

; There would be a problem is max. power were developed at 3000RPM as ; would give capstan drum of 50mm dia - too small for 12mm rope. So ; couldn't have that.

; OK - leaving this late on 15 April 2010, what needs to be known more ; exactly is clear - the power and RPM of usable motors.

; However - 10m per minute maximum might be too slow - might want ; higher than that. If max. power at 3000RPM (other guess) would want ; 6HP output - something like 10HP input...

Rich Smith

Richard Smith fired this volley in news: snipped-for-privacy@Richard-Smiths-Computer.local:

It's been SED that a Unix accent is acceptable, but people with a LISP sound weird!

LLoyd

FWIW, there are numerous small gasoline engines that have a gear reduction built into the side of the engine block/crankcase, or bolted on.

Honda GX series has ideal rev-range at the crankshaft.

50:1 worm-gearbox gives one rev per second at the capstan at full revs

- is just right? Rich Smith

Hellifiknow.. I'm just reading here.. never had a craft that needed a capstan, and not making recommendations.

Note that gear drives may or may not tolerate radial loads on the output. Better find out on yours before the project gets too far along. JR Dweller in the cellar

Richard Smith wrote:

You poor soul - you don't know what you're missing out on!

It's all there for you - jokes about the real reason you do this stuff, bets that you are going to get arrested for "suspicious behaviour" before the end of the year, people stopping and looking nervously from afar when you are setting up experimental test-rigs, other friends who use capstans, block and tackles and know 20+ knots and splices off by heart

Be encouraged

Rich Smith

That's one of the issues, JR. Better not pass on loads from the tension of the rope against the capstan to the gearbox. Got to send those loads - got to be through some bearings - to a structural frame in which the gearbox is mounted, subjecting itself only to torque.

Got any ideas?

Rich Smith

JR North writes:

A lot of gearboxes are used to run chain drives or v-belts, which can have a substantial sideways load, so it might be just fine? I'd check the manufacturer spec sheets--if you can find any--before I went ahead and designed a jackshaft, frame, and lovejoy or double-row chain coupling. Or maybe specsheets for several other similar gearboxes from Boston Gear, Marvin, Eaton, etc. to see what the range of typical values is.

But if you have to design something, they're pretty straightforward. A chain coupling would probably hold up better than a lovejoy (3-piece flexible coupling) under the higher torque loads on the output side. SurplusCenter.com has both styles of couplings in the states, you may find their pages useful for figuring out what you need before you try to find them locally.

I'll also note that most of the similar gearboxes they have listed for sale are rated only up to 1800rpm input. With a typical 3600rpm small gas engine, you'll either need 2:1 input pulleys, or to find one with a reducer box (some extend the camshaft out the side case for 2:1 auxiliary output, some come with a 6:1 ring gear setup, I'm not sure if there is an add-on 2:1 gearbox available, but all the gearbox versions I've seen are harder to find and more expensive--if you're scrounging, 2:1 pulleys are easier and give you more cheap/available engines to pick from).

Hope that helps, --Glenn Lyford

It wasn't until here in your article that I realized that you were talking about the kind of capstan used on sailing ships instead of a lathe turret, which is sometimes called a capstan in the UK. I think that the distinction is between the bed turret (such as I have on my

12x24" Clausing which can be interchanged with the standard tailstock) and the built-in turret which lives on a carriage and is coupled to the feed rods for power.

Well ... the ones which I have seen on old square-riggers (the kind powered by a bunch of people marching around them pushing on bars) were iron castings, including the ridges to help it couple to chain instead of just rope. (It was used for raising the anchor as well as for many other things needing a long pull with lots of tension.

Now, I'll stop, because the ones which I have seen were purely muscle powered. :-)

Enjoy, DoN.

Hi DoN and

DoN - that's right - of the nautical type. There is a capstan lathe which I guess is named because the massive rotating tool-holder is rather capstan-like - seen a derelict World-War-2 one in a thoroughly modern (!) Fab. shop I worked in (a new Rumanian guy was speechless with shock as he sat at his first tea-break - he'd never seen anything like this place in Western Europe or even in Rumania).

Glenn - speed - yes - assume electric motor was 1500RPM (50Hz power here) - so running gearbox at up to double that. Problem? Don't know. Reduction ratio is already low enough at 50:1

I've thought of a design where the tube which becomes the capstan barrel is welded to a disk of slightly larger dia. than the tube / barrel, then the periphery of the disk protruding beyond the tube becomes the rotating part of a big all-around sliding bearing. The load is transfered to a steel plate "deck" under which the gearbox is mounted - tried to sketch in ASCII text

|| || \\ // \\ // \\ // // \\ // \\ // \\ || || __ || X || __ | ===========X========== | --------------------- X ---------------------- X

Then the drive shaft from the gearbox only has to keyway to the disk (it serves that purpose as well - picking up the drive)

Another question for everyone...

Previous calculations I sketched out suggest a capstan barrel diameter at the waist of 150mm (6"). The whole design is intended to be matched to 12mm (1/2") rope [3-strand layed polypropylene - which should have breaking strength of 2.2Tonnes-force]. So is 150mm (6") the right capstan waist size for 12mm (1/2") rope?

Rich Smith

I would make sure that the capstan will not spin free and drop the load should the capstan drive system break under load. Even if the load is of no importance, whoever was pulling on the rope when the capstan failed could be pulled into the capstan and mangled.

There must be many patents on better designs for marine capstans. Some research may be helpful.

Joe Gwinn

On the other hand, there are times when that "drop" capability had to be designed in.

Lowering an anchor at capstan speeds could take half the day.

When you are located where you want to be, you want that anchor to hit bottom as soon as possible.

Yes, usually there is a clutch built into the hub, but more and more the high end anchor winches are power up - power down and if there is a clutch it is seldom used.

Of course that usually applies only to the "gypsy" side. the capstan side is usually keyed to the shaft. If a horizontal winch :-) John B. Slocomb (johnbslocombatgmaildotcom)