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Steel drive cable on 20" SkyWatcher 500p Dobsonian

Dob Equipment Reflector
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11 replies to this topic

#1 SteveNZ

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Posted 04 December 2022 - 02:33 PM

Hi Everyone,

 

I've recently taken delivery of a 20" SkyWatcher and have some concerns about the steel drive cable.

 

I'm not mechanically minded, so would really appreciate any opinions and ideas from someone who has experience with this sort of thing.

 

1) The cable I have is fraying in 4 or five places along it's length. I've cut off (the 1cm long) frayed ends. The cable works fire, and I only notice the frayed wire when it stabs me in the fingers each time I thread the cable around the pulleys!

However my main concern is that it will continue to fray and one day a strand of wire will come loose and jam itself somewhere during use and cause major damage.

Has anyone seen this sort of thing before? Is it usual? Is it a concern? Should I get the cable replaced (will it do any good to do so)?

 

2) This steel cable takes some very tight bends around the pulley system and is under a lot of tension in use as it moves the OTA up and down in elevation. I'm wondering if it's OK to leave the cable threaded between viewing sessions, with the cable fed around all those sharp bends for days / weeks / months? Will this damage the cable if it's left like that? (the smaller pulleys it wraps around are around 10mm diameter). Or, should I take the primary mirror off the base and unthread the cable each time I use the telescope?

 

I've attached a picture where you can see the path that the cable takes around the pulleys.

 

Any advice greatly appreciated!

 

- Steve

New Zealand
 

 

Attached Thumbnails

  • SkyWatcher500p-base-with-cable.png


#2 SoCalPaul

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Posted 04 December 2022 - 02:41 PM

Hi Everyone,

 

I've recently taken delivery of a 20" SkyWatcher and have some concerns about the steel drive cable.

 

I'm not mechanically minded, so would really appreciate any opinions and ideas from someone who has experience with this sort of thing.

 

1) The cable I have is fraying in 4 or five places along it's length. I've cut off (the 1cm long) frayed ends. The cable works fire, and I only notice the frayed wire when it stabs me in the fingers each time I thread the cable around the pulleys!

However my main concern is that it will continue to fray and one day a strand of wire will come loose and jam itself somewhere during use and cause major damage.

Has anyone seen this sort of thing before? Is it usual? Is it a concern? Should I get the cable replaced (will it do any good to do so)?

 

2) This steel cable takes some very tight bends around the pulley system and is under a lot of tension in use as it moves the OTA up and down in elevation. I'm wondering if it's OK to leave the cable threaded between viewing sessions, with the cable fed around all those sharp bends for days / weeks / months? Will this damage the cable if it's left like that? (the smaller pulleys it wraps around are around 10mm diameter). Or, should I take the primary mirror off the base and unthread the cable each time I use the telescope?

 

I've attached a picture where you can see the path that the cable takes around the pulleys.

 

Any advice greatly appreciated!

 

- Steve

New Zealand
 

Hi Steve,

 

I can't address #2, although I share your concern that this could result in a permanent "bump" (crimp) in the cable, where it is tightly bent around those smaller pulleys.

 

But with regard to #1, I will share my experience from cycling. I have had my derailleur cables fray inside the brake handle/shifter housing, and it has destroyed the internals, requiring complete replacement. I recommend contacting SW support and see what they have to say.

 

Clear skies,

Paul



#3 havasman

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Posted 04 December 2022 - 03:36 PM

In our manufacturing plants we had lots of different lifting devices, many that had braided steel cables in critical spots. I always considered a frayed cable was caused by something that had not been fixed and that they would continue and probably accelerate toward failure. With several different bad spots along the length, there's likely no emergency need for speed but replacement seems indicated. You might even get smoother action from the system. It would probably be good to store for longer periods with the cables relaxed, particularly if there's a correlation between the wear spots and pulley locations. Seems like that machine should have had a lever action of some kind to partially release the tension. But good cables should be strong and mirror handling carries some risk too. It might be worth looking at the surfaces on the pulleys to see that they're not oddly worn or very rough.



#4 drd715

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Posted 04 December 2022 - 06:35 PM

Replace the cables and don't worry about the long term effects on storing the cables in place.


Edited by drd715, 04 December 2022 - 06:35 PM.

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#5 SteveNZ

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Posted 05 December 2022 - 04:24 PM

Hi Steve,

 

I can't address #2, although I share your concern that this could result in a permanent "bump" (crimp) in the cable, where it is tightly bent around those smaller pulleys.

 

But with regard to #1, I will share my experience from cycling. I have had my derailleur cables fray inside the brake handle/shifter housing, and it has destroyed the internals, requiring complete replacement. I recommend contacting SW support and see what they have to say.

 

Clear skies,

Paul

Hi Paul,

Thank you for sharing your experience, it does sound like the potential for something nasty to happen. I have contacted the retailer and he's forwarding my concerns to SW.

In the short term, I thinking about getting a replacement cable, I'm thinking that a marine hardware place may be able to make one up for me - we have a strong boat building industry here in New Zealand wink.png



#6 SteveNZ

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Posted 05 December 2022 - 04:30 PM

In our manufacturing plants we had lots of different lifting devices, many that had braided steel cables in critical spots. I always considered a frayed cable was caused by something that had not been fixed and that they would continue and probably accelerate toward failure. With several different bad spots along the length, there's likely no emergency need for speed but replacement seems indicated. You might even get smoother action from the system. It would probably be good to store for longer periods with the cables relaxed, particularly if there's a correlation between the wear spots and pulley locations. Seems like that machine should have had a lever action of some kind to partially release the tension. But good cables should be strong and mirror handling carries some risk too. It might be worth looking at the surfaces on the pulleys to see that they're not oddly worn or very rough.

In my case, the cable is brand new, so unless something on the pulleys is really, really bad enough to cause this fraying after a day or two while I'm assembling and trialling the telescope I would have thought it was the cable at fault? 

I'll take your advice and carefully check the pulleys - thank you.



#7 SteveNZ

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Posted 05 December 2022 - 04:36 PM

Oh, and by the way - last night was this telescope's first light ! ! ! I'm just so happy, the views with a 20" dob are just stunning ! The detail in nebula, the brightness of DSOs with high magnification eyepieces . . . and this was from my front yard in the suburbs. Just . . . wow  smile.gif


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#8 Waynosworld

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Posted 05 December 2022 - 05:38 PM

I would find out why it is fraying, my Hubble Optics had this issue and it was because the drive pully was not grooved to keep the cable from overlapping itself, when it pulled the cable out from under the cable above it, it would make a tinging noise and fray the cable, I have since removed all of it and use it manually now, it is way easier/faster for me to use this way.


Edited by Waynosworld, 05 December 2022 - 05:39 PM.


#9 Ivan Maly

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Posted 14 December 2022 - 07:25 AM

Steve, neither is a concern. I've had this scope for 4 years now. My only advice is (I hope it is not too late) not to cut anything off. Wear work gloves when you put the "mirror box" on and off so as not to be poked by the cable ends.



#10 briansalomon1

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Posted 14 December 2022 - 12:20 PM

It isn't clear to me if you need to thread the cable trough the guide pulleys on a regular basis. If it doesn't interfere with functionality, heat shrink on the cable ends will make handling the cable more comfortable. I just use a lighter to shrink mine.

 

https://www.amazon.c...,aps,222&sr=8-5

 

I've found that smaller diameter cable is far more resilient than larger cable when routed through small radius turns.

 

The motors in the drive mechanism of your telescope should have a part number you can use to track down the specifications. You can convert the HP rating to pounds of force here https://www.convert-...p?type=leistung A 1/8hp motor (you surely have smaller that 1/8hp) will make 68 foot pounds of force.

 

This stainless cable is rated for 340lbs. https://www.harborfr...rope-61784.html



#11 SteveNZ

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Posted 03 January 2023 - 03:13 PM

Thank you everyone for your advice, it was greatly appreciated.

 

I have closely inspected the path that the cable takes and can't find any issues that could have caused the fraying. Although, early on I noticed that some tiny recessed bolts on the small pulleys were loose and were protruding slightly, so I tightened those - perhaps that's what caused the fraying, before those bolts were tightened?

 

Since then I've received a brand new cable from my supplier, so everything should be ok now. Also I've had more experience with the setup and operation so I should be in a better position to understand any further issues that may arise.

 

Thanks again for your help!



#12 Gaseous

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Posted 06 January 2023 - 05:12 PM

Thank you everyone for your advice, it was greatly appreciated.

 

I have closely inspected the path that the cable takes and can't find any issues that could have caused the fraying. Although, early on I noticed that some tiny recessed bolts on the small pulleys were loose and were protruding slightly, so I tightened those - perhaps that's what caused the fraying, before those bolts were tightened?

 

Since then I've received a brand new cable from my supplier, so everything should be ok now. Also I've had more experience with the setup and operation so I should be in a better position to understand any further issues that may arise.

 

Thanks again for your help!

Glad you got your issues sorted Steve. I've had the same scope since mid-2018 (across the ditch from you) and the cable has been left attached the whole time with no ill effects - it's a pain having to connect it every session. As others have said, you gotta watch those spiky ends though!

 

Cheers,

Pat.
 




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