Obs shutter drive: winch slipping: fix ?

[skybadger]

Hi all 

I have put in a shutter winch system into the obbo. It uses wire rope running over rollers on the dome slit side and returns via cable guide to the winch motor at the bottom. The design is an endless rope, for which the winch bobbin behaves like a sail winch and spools the wire in at the top and out at the bottom.  However , I'm getting wire overlay, even with the in and out feeds staggered and this leads to it stalling. I did originally have a 'u' cross-section on the bobbin but that also slipped due to lack of turns. 

See the picture at top . It shows the bobbin with 3 loops of wire wrapped around it to be a friction cable drive. Diameter is about 50 mm. The wire is 1.6mm bike wire.

And this is the wider view. 

Anyone have a view on how to prevent the wire overlaying on itself . Do I just need a longer bobbin ?

Cheers 

Mike

 

[Oddsocks]

Hi Mike.

Sorry to disappoint you but with your current design there is no way to prevent the wire loops from travelling across the pulley and crossing.

Having some experience of wire rope driven reciprocating machinery we would normally use a large diameter drum with the rope fixed to the drum in the centre of the span and with the equipment in the fully closed position one side of the drums centre position would have no coils of rope while the opposite side would have enough coils of rope to allow the equipment to reach its max extension. As the equipment operates the drum rotates and the rope uncoils from loaded side while the rope coils itself on the previously empty side. There is no need for cable guides with this method and indeed they would be a hinderence to allowing the rope to smoothly wind and unwind on the drum.

Using a centre fixed rope drum means having a large enough diameter drum and enough rope so that the entire movement range can be accomodated without the rope ever overlapping itself, the entire operating range has to be contained in a single layer of rope otherwise it is impossible to maintain a constant tension on the rope and tangles or snags are inevitable. 

The disadvantge of a drum is that a) it is relatively large and heavy and b) to keep the coils laying flat the rope tension has to be kept constant across the full range of movement which is difficult with a flexible GRP dome.

With your current design you need to keep sufficient tension on the rope to prevent slipping on the pulley and this will unfortunately cause the rope to travel across the pulley as it rotates, there is no way to mitigate this in this design.

I think you will find a lightweight chain and sprocket system will offer you the simplest method of shutter actuation, this is the method that I have in my Pulsar dome though I replaced the plastic chain that Pulsar supplied, and kept breaking, with a light weight composite stainless steel and polymer chain.

An alternative to a chain is to use a toothed belt though these are more difficult to implement as flex in the dome means it is hard to keep positive tension between belt and pulley leading to the belt ‘jumping’ teeth if the belt sags as the dome flexes during shutter movement. To use a toothed belt successfully needs small jockey wheels either side of the pulley to keep the belt pressed tightly against the pulley and stop the belt lifting as the belt tension changes over the full range of shutter operation.

HTH.

William.

[Gina]

I had the same dilemma with the drive to my roll-off-roof.  Bicycle chain was the answer, with sprockets.

[Peter Drew]

+1 for chain and sprocket drives for up and over doors. I moved to bi-parting doors as I found them to be more reliable.   ?

[Rusted]

The only obvious way to ensure a flat cable lay is a heart shaped cam and actuating arm with double roller cable guide.

Like an old fashioned, Singer sewing machine, spool winder. The cam has to be worm geared.

It quickly gets rather complicated and may not stand up to heavy cable loads.

I should have added that a cable drive ideally needs a V-pinch roller for grip.

That probably means one near, full turn of cable but no more.

The more wrap the better but the cable cannot cross itself.

[sloz1664]

I would like to automate my shutter on my old Pulsar dome and wire rope was one of the methods I was looking at, together with chain and sprockets and belt driven.n Although I was discounting belt driven, for the reason William had pointed out, slippage and also cost. 

Regards the wire method. As previously pointed out, the wire will want to run laterally across the bobbin, which equates to the thickness of the wire per revolution of the bobbin. It will run laterally in one direction when opening the shutter and run laterally in the opposite direction when shutting the dome. As your wire rope is 1.6mm dia; you will need to add this to each side of your bobbin beyond your initial three coils for each revolution of your bobbin. calculate how many revolutions the bobbin will make for one full shutter travel. You will not be able to restrict the wire travel whilst it travels across the bobbin.

5 hours ago, Oddsocks said:

 I think you will find a lightweight chain and sprocket system will offer you the simplest method of shutter actuation, this is the method that I have in my Pulsar dome though I replaced the plastic chain that Pulsar supplied, and kept breaking, with a light weight composite stainless steel and polymer chain.

William,

Do you have a contact for the  light weight composite stainless steel and polymer chain.

Steve

[Oddsocks]

2 hours ago, sloz1664 said:

William,

Do you have a contact for the  light weight composite stainless steel and polymer chain.

Steve

Hi Steve.

The chain was a supplied by BRT bearings here in the UK, the contact was Mr Gordon Andrews: 

brt.sales@brt-group.com

Web site:

http://www.brt-bearings.com/index.php

You should prepare yourself for a shock though, the chain is a specialist product and very expensive, £200, for a minimum 5 meter length and a couple of spare joiner links.

They do not keep it in stock and it is imported from Germany on special order which takes just a couple of days.

If you want to follow that path the chain is called RENOLD ISO-06B-1 SYNO-PC SIMPLEX 3/8' PITCH.

Details here at a UK importer (but they don't supply direct to the public hence going to BRT Bearings for the purchase): 

http://www.renold.com/products/industrial-transmission-chain/renoldsyno/renold-syno-pc-chain/

This chain type was chosen over a conventional all metal chain because it requires absolutely no lubrication -ever- so it does not attract dust and is many times stronger than an all-plastic chain. If it does need a clean this is done by just washing it in isopropyl alcohol. A composite polymer chain offers the best compromise between weight, noise and strength, but as mentioned above, at a hefty price.

I also changed the PULSAR supplied drive sprocket, which was plain steel and going rusty after just a few months of use, with a stainless steel sprocket of the same size from Cross Morse in Birmingham, they also drilled the sprocket shoulder for me and tapped an M4 thread for the locking screw to fix the sprocket to the drive motor shaft.

sales@crossmorse.com

http://www.cross-morse.co.uk/index.asp

The sprocket was not expensive, type 06B1-12T, cost ~ £18

If you don't mind the noisier operation and occasional lubrication and cleaning required then an all stainless-steel chain is a good choice for the observatory, being around half the weight of a conventional plain steel chain and no rusting issues, but even that is expensive compared to a plain steel chain. RS carry a range of stainless-steel type 06B-1 simplex chains that cost (for 5mtr) from ~£150 up to ~£350, looked at in that light, the price difference between an all stainless steel chain and the composite stainless and polymer chain is not so very different.

https://uk.rs-online.com/web/c/pneumatics-hydraulics-power-transmission/power-transmission-roller-chains-accessories/roller-chains/?sra=p&applied-dimensions=4294534454

The type of chain, 06B-1, 06B-2 refers to the pitch standard so once you decide what chain size and pitch standard to go for you can specify your sprockets, number of teeth, drive shaft diameter and shoulder size etc.

A few photos attached below showing the old all-plastic chain that PULSAR supplied and the Stainless-Steel-Polymer composite chain from RENOLD that I fitted as a replacement.

One of the images shows the addition of a sprung chain tensioner that was needed to take up the variable chain slack as the dome flexes over the full range of shutter movement. It was this variable slack in the chain during opening and closing of the shutter that was causing the old all-plastic chain to keep breaking as the chain would sag and catch on the PULSAR pulley supports or jam in the sprocket teeth on the motor, the sprung chain tensioner was added to keep the new chain taught and prevent the same thing occurring. Although the composite chain is strong enough not to break the risk of the chain jamming around the drive sprocket and stalling the motor remained and as I operate the dome remotely (from abroad) reliability was a major driver.

 

HTH.

William.

Original all-plastic shutter chain supplied with the dome:

New composite stainless-steel and polymer RENOLD SYNO-PC chain:

Composite chain, view of PULSAR guide pulleys.

Sprung chain tensioner added:

Full shutter open view with composite chain:

 

[sloz1664]

Thanks so much for the comprehensive information you have supplied. Nothing phases me on the cost of specialised products. It cost me more or less the same for the Dome rotational drive belt attached to my Obsy.  I'll certainly look at what I need, now I have contact details. It's annoying that Pulsar offer a chain mechanism that is not fit for purpose, especially with the extortionate cost of the shutter system.

Again thanks for the info.

Steve

[skybadger]

Thanks all for the input.

I'm surprised that the wire is expected to ride sideways since this is the method used in boat winches and such.

I spent yesterday turning a new profile pulley which is winch bobbin shape, and hope to test that in the workshop shortly, to see if I can get the loops to move along into the valley and prevent snagging. After that I shall have to decide on belt or chain.

[hughgilhespie]

Hi,

I used stainless steel chain to drive the shutter on my 2.2 metre Pulsar.  I actually used 1/2 inch chain but that was overkill. The 3/8 inch Dunlop stainless steel chain is available in a 5 metre length from Bearing Boys at about £92 including VAT. Link below. They also have the connecting links and matching stainless steel sprockets but you will need to drill/ream  these to suit your motor shaft diameter and also drill and tap for a grub screw to secure the sprocket to the motor. 

https://www.bearingboys.co.uk/Simplex-Chains/06B1SS-Roller-Chain-5-Meter-Box--Stainless-Steel-Dunlop-133466-p?gclid=Cj0KCQjw45_bBRD_ARIsAJ6wUXS4USTrb6DZou5oVmtKDDfylzsQeBF27YdO6r9zrdD9EuFAF8G-nD8aAnnpEALw_wcB

The stainless steel chain is a lot cheaper than William's composite but of course it does need lubricating. Probably only once a year or less - how often do you open / close your dome shutter?

HTH

Hugh

[hughgilhespie]

A bit more information. As William said, you do need some way of keeping the chain tensioned for reliable operation. My answer for my homebrew system was to mount the motor on a swing mounting. The mounting is actually a length of aluminium angle. A strong spring stopped the motor from 'climbing the chain'. In the picture below the right hand bolt is the pivot bolt. The other two bolts are to keep the pivot plate tight to the backing plate - i.e. confine the rotation to one plane and also to act as limit stops - there are slots cut into the mounting plate to allow it to rotate. All the bolts have teflon washers front and back to keep the friction down. It was a simple design but it works well.

Hugh

[Rusted]

10 speed bike chains are usually coated with an anti-rust finish. Colours too, if it matters.

Sprockets are readily available but with a big hole in the middle, left or right hand threads and/or oddly splined.

Simply Bearings [UK] lists various sizes of "pilot hole" sprockets in 1/2" pitch.. Single, double and triple.

https://simplybearings.co.uk/shop/Engineering-Parts-Chains,-Links,-Sprockets-&-Platewheels-Sprockets-Pilot-Bore-Sprockets-Simplex-Pilot-Bore-Sprockets/c4713_5487_5510_5511_7747/index.html

I once used a left side, bottom bracket cup [same thread as screw-on sprockets and blocks] with a small sprocket to drive a mirror making machine.

Or use a whole, threaded bike hub and sprocket if you need bearing support for a layshaft.

[Stub Mandrel]

Try using a single or double loop rather than triple.

If it slips, try fitting a rubber 'tyre' round the pulley. A slice of bike inner tube should work for a good while as it's not continuous use.

[skybadger]

Thanks Hugh and Stub mandrel.

I will try the new profile pulley. According to the MAPUG news groups, Doc G had a dome rotation solution of this type working in the 90s . The difference seems to be to use a wide roller for the pulley and a widely separated feed. Some boat winches recommend a feed angle of ~8°. One of the tricks is to get the fore and aft tension right to prevent slip.

My approach is to:

Change the pulley profile to more of a V.(done, to test)

Reduce the pulley clearance from the feed.(done, to test)

Try changing the number of coils on the pulley. 

Try using an extra wide un flanged pulley.

If that doesn't work, I also have some 1/4" chain from large HP printers I can try, with associated plastic drive sprockets. 

Finally, if it comes to it, I can modify the existing wire roller brackets to use timing belt, assuming pinch rollers around the motor pulley.

Anyone recommend a source of inline remaining springs, 60mm or so long, 10kg/cm force constant ?

Ta

Mike

 

[Oddsocks]

On 06/08/2018 at 08:11, skybadger said:

I'm surprised that the wire is expected to ride sideways since this is the method used in boat winches and such.

Hi Mike.

I used to race sailing boats years ago. A sheet winch is designed to operate one way, the bottom of the winch drum is conical and this is the where the sheet enters, then the main body part of the drum is flat and smooth, this is where two or three turns of sheet are wound, the top of the drum has a narrow shoulder and on the bigger winches a pair of snap jaws, this is where the free end of the sheet leaves the winch.

In operation, as the winch is rotated, new sheet enters the drum on the conical part so it has a larger diameter than the turns on the body, this continually feeds in a little slack to the sheet turns around the body so that new sheet entering on the cone forces the turns around the body to slip upwards. This is why you don’t (normally) see the coil migrating across the winch and why you sometimes end up with a tangle if the sheet becomes impregnated with sand or grit that prevents the sheet from slipping upwards across the body.

New sheet entering the winch always has to be coming from below the lowest part of the conical section, if it approaches from above the cone the sheet won’t be able to slip upwards on the body and the winch will jam.

On winches without snap-jaws tension has to be kept on the free end of the sheet at all times otherwise the winch stops working, the coils don’t have enough purchase on the drum by themselves to maintain a grip, the winch drum may turn but the coils will slip. On winches with snap-jaws, the jaws grip the final exit turn of the sheet and provide the constant tension needed for the winch to work and free the operator from having to hold on to the free end of the sheet and provide a constant ‘pull’.

In practice the snap-jaws were as much a curse as a blessing and caused me to capsize more times than I care to remember as I often couldn’t release the sheet from snap-jaws fast enough when the wind picked-up and I would be taking an early bath before you knew it. In the small boats I used to sail I gave up on the snap jaws on blustery days and just hung on to the free end of the sheet and stayed dry.

To release the sheet the winch is not rotated in the opposite direction as it would not work and the coils would wind off the top of the drum, instead, the drum is locked by a one-way ratchet and the tension on the free end of the sheet relaxed to allow the sheet to slip around the stationary body.

Not so easy to describe in words but there are loads of YouTube videos out there showing a sheet winch in operation.

William.

[skybadger]

Hi Will,

Thanks for the input.

I too used to sail and use winches, so I'm familiar with what you mention. I've put a bevel on the sides of the pulley now and am looking for time to test. 

Cheers

Mike

 

[Stub Mandrel]

Won't such a bevel only work for one direction of pull?

[skybadger]

Hi Stub

When sailing, you load the winch in the direction you need to haul and rely, as Will said, on the in feed slope to push the coils up towards the parallel part of the bobbin where all the friction takes place. So I now have a pulley with sloped sides at each side and parallel centre. The in feeds will land the cable on the slope of the pulley and push the coils  along. For both directions of feed.

Well, that's my theory. 

I'm rapidly running out of time for this before my hols, so I might have to report results back in a fortnight.

Cheers

Mike.

 

[Peter Drew]

I used chain drives for up and over doors when I used to make domes commercially. I bent 5/8" x 1/8" guage aluminium channel, legs inwards to suit the inside profile of the door. The 3/8 chain was next squeezed into the channel which firmly held it in place. The unit was then fitted centrally to the door which increased its stiffness. Drive was via a  sprocket connected to an auto wiper motor fitted to the crosspiece at the apex of the dome opening.    ?

[skybadger]

Hi Peter

I have considered similar. Due to the lower shutter sliding underneath the higher, larger shutter, adding the guide slot is hard from a clearance point of view. I don't even have 1/4", so I have considered belting and some low profile guides to keep the belt close to the circumference. However having just pm'd Steve 1962, I think he hauls across the chord. Not yet sure how that closure works though.

I have to haul from the bottom of the lower shutter and it collects the upper as it rises. 

I shall have to think if there's space and how to arrange it. 

 

Many thanks

Mike.

 

 

[skybadger]

Mm, I just RE-read and I think I misunderstood. The chain remains tight and fixed in the channel doesn't it, rather than act as a guide for a moving chain. A bit like nailing it to the inside of the shutter. Is that right ?

M