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Author Topic: Digihat replacement for DCC'ing older Farish locos  (Read 1186 times)

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Offline NinOz

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Re: Digihat replacement for DCC'ing older Farish locos
« Reply #15 on: February 06, 2017, 01:16:02 pm »
The method works but if the loco is run continuously causing the brushes to heat the heat shrink degrades and can eventually fail.  I use sound decoders and I just wont risk a £90 decoder for a few pounds of parts.
???
Some heat shrink continuous max rated
PTFE  180C+  (have seen quoted to 260C)
Viton 220C
Polyolefin 135C
Elastomer  150C
Silicone 190C
PVC 105C  (careful with PVC as it slowly degrades in service over 10 to 20 years, even at low temperatures)
Kynar 175C  (I think the digihat is made of Kynar)

Currently looking for some short lengths of ultra thin PTFE shrink tube (0.05mm wall thickness), can only find suitable one available in 100' roll.   May have found a suitable supply in China.

CFJ


To be called pompous and arrogant - hell of a come down.
I tried so hard to be snobbish and haughty.

Online Snowwolflair

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Re: Digihat replacement for DCC'ing older Farish locos
« Reply #16 on: February 06, 2017, 01:22:44 pm »
The method works but if the loco is run continuously causing the brushes to heat the heat shrink degrades and can eventually fail.  I use sound decoders and I just wont risk a £90 decoder for a few pounds of parts.
???
Some heat shrink continuous max rated
PTFE  180C+  (have seen quoted to 260C)
Viton 220C
Polyolefin 135C
Elastomer  150C
Silicone 190C
PVC 105C  (careful with PVC as it slowly degrades in service over 10 to 20 years, even at low temperatures)
Kynar 175C  (I think the digihat is made of Kynar)

Currently looking for some short lengths of ultra thin PTFE shrink tube (0.05mm wall thickness), can only find suitable one available in 100' roll.   May have found a suitable supply in China.

CFJ

Its not what its made of its the fact it keeps shrinking and eventually the brass is bare and touches the chassis.

A non shrink exact fit thin tube is the way to go but that does not help the underside of the lip touching the chassis unless you also use a nylon or similar washer.  Also the tube if it is not a tight fit crawls down the brush holder exposing the other end, and again shorting to the chassis.


Offline Only Me

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Re: Digihat replacement for DCC'ing older Farish locos
« Reply #17 on: February 06, 2017, 01:55:46 pm »
corrected... would swear that said £9 earlier today!



Offline NinOz

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Re: Digihat replacement for DCC'ing older Farish locos
« Reply #18 on: February 06, 2017, 02:16:41 pm »
Its not what its made of its the fact it keeps shrinking and eventually the brass is bare and touches the chassis.

A non shrink exact fit thin tube is the way to go but that does not help the underside of the lip touching the chassis unless you also use a nylon or similar washer.  Also the tube if it is not a tight fit crawls down the brush holder exposing the other end, and again shorting to the chassis.

Thin bit of plastic (couple of thou) or kapton tape takes care of top "lip" insulation.  See my method above. 
One must be careful about the thickness of the insulation used as this will withdraw the end of the brass tube away from the commutator allowing the brush to have more wriggle room with consequent faster wear.

Shrink tube shrinks mostly radially not longitudinally and it would do most of that when initially shrunk as there would be little resistance to movement along the brush holder.  Amount of residual shrinkage available depends on how close the polymer is to the relaxed state after shrinking so one would start with a tube whose shrunk relaxed diameter is slightly smaller than the brass tube.

Movement of an insulating tube is prevented by the use of a substance I call "glue" which is used to fix the insulating tube to the brush holder or the chassis block.

I have never experienced or seen any such failures nor heard of any examples of such failures among those who use the shrink tube method.  Only seen warnings of a possibility; thought experiments.

Good thing is that with newer models this is no longer a consideration. 
Only ever converted a few of my old farish as they are such poor runners to start with, even put a flywheel in a Class 47 but that made minuscule improvement, much preferred to put decoders into my Kato and Atlas locos which still run superbly.

CFJ
To be called pompous and arrogant - hell of a come down.
I tried so hard to be snobbish and haughty.

Online Snowwolflair

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Re: Digihat replacement for DCC'ing older Farish locos
« Reply #19 on: February 06, 2017, 02:21:58 pm »
Its not what its made of its the fact it keeps shrinking and eventually the brass is bare and touches the chassis.

A non shrink exact fit thin tube is the way to go but that does not help the underside of the lip touching the chassis unless you also use a nylon or similar washer.  Also the tube if it is not a tight fit crawls down the brush holder exposing the other end, and again shorting to the chassis.

Thin bit of plastic (couple of thou) or kapton tape takes care of top "lip" insulation.  See my method above. 
One must be careful about the thickness of the insulation used as this will withdraw the end of the brass tube away from the commutator allowing the brush to have more wriggle room with consequent faster wear.

Shrink tube shrinks mostly radially not longitudinally and it would do most of that when initially shrunk as there would be little resistance to movement along the brush holder.  Amount of residual shrinkage available depends on how close the polymer is to the relaxed state after shrinking so one would start with a tube whose shrunk relaxed diameter is slightly smaller than the brass tube.

Movement of an insulating tube is prevented by the use of a substance I call "glue" which is used to fix the insulating tube to the brush holder or the chassis block.

I have never experienced or seen any such failures nor heard of any examples of such failures among those who use the shrink tube method.  Only seen warnings of a possibility; thought experiments.

Good thing is that with newer models this is no longer a consideration. 
Only ever converted a few of my old farish as they are such poor runners to start with, even put a flywheel in a Class 47 but that made minuscule improvement, much preferred to put decoders into my Kato and Atlas locos which still run superbly.

CFJ

In 22 years must have converted over 350 old Farish steamers for myself and club members. Seen enough to know what happens.  :D

Offline Madrigal

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Re: Digihat replacement for DCC'ing older Farish locos
« Reply #20 on: February 19, 2018, 12:59:53 am »
Just a quick message on this post, the Marwick Brush has been very successfull, the technical specification is as follows "The ‘MarWick’ Brush Insulating Sleeve is manufactured from a Polyoxymethylene copolymer (POM-C) also known as Acetal. It is an engineering thermoplastic used in precision parts requiring high stiffness, low friction, and excellent dimensional stability. POM is characterized by its high strength, hardness and rigidity. It has a melting point of 175°C."

The sleeve is designed to give an internal diameter which easily accepts a standard brush without filing and has now been used in over 100 conversions without complaint, you can view a series of videos showing models converted using the Marwick brush here https://www.youtube.com/playlist?list=PLQZGvJCD0M7RYQpd4ndq3sfCXCD_hbNbz

 

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