OK, just taken one apart to familiarise my self. I had assumed the thrust pad was in the bottom of the cap, I see it's just held there. So that makes the thrust pad as we know it, attached to the shaft. I get the terminology now !
I have some made out of PTFE but can't determine if that was tested ? and haven't tried them myself.
Also as the unit it Japanese, the ball is probably 7mm and that would account for the discrepancy ? http://www.ebay.com/itm/350891628269
Dave
I have some made out of PTFE but can't determine if that was tested ? and haven't tried them myself.
Also as the unit it Japanese, the ball is probably 7mm and that would account for the discrepancy ? http://www.ebay.com/itm/350891628269
Dave
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Thanks for the link, Dave!
Maybe you can share a little bit of knowledge of how these bearings work.
As I understand the ball should freely rotate in the housing "tube/cylinder", so it is better to have a little bit smaller diameter of the ball then ID of the tube, right?
It looks to me that 9/32 is a bit to tight if you need to press it in. However, wouldn't 7mm be to small? Ideally you want a contact spot to be a point, you don't want the boll rolling on the pads.
Maybe you guys can comment on how tight 9/32 is fitting there and how hard you need to push it in? Another thing is a lube, you need some gap for it as well.
If it is not the case, how it is written in the link 6L6 gave, and you want the ball to stuck, than 6/32 looks like a better choice... I am not sure how the deliver oil underneath and if you need it...
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Well, all plastics flow (well cross linked ones do it less), also their elasticity is temperature dependent. So the difference is mostly quantitative, so, I guess jpjones3318 says that it flows more at lower temp. and on faster time scales. I haven't looked into PTFE viscoelastic properties, though... and after all i am a theoretician, not an engineer 
Let's hope jpjones can fill us in. I really would like to know and google isn't helping at all, although this came up : Toray PTFE TEFLON® Bearings, DuPont TEFLON® brand polymer,
And then "PTFE polymer will cold flow at pressures of 2,000 psi which limits the polymers usefulness as a bearing material" which is a very high pressure !
Then Top 5 Materials for Plastic Bearings Used on Metal Surfaces
And : 2.Load bearing
A common misconception relating to the load bearing capacity of PTFE leads many machine tool builders to write-off PTFE as a wear pad material. The assumption is that phosphor-bronze, being a metallic material, is the only option strong enough to take the load of heavy moving parts.
In truth – PTFE has a compressive strength of at least 135-140Kg per square cm. This implies that a 100mm x 100mm plate would be able to withstand 13.5-14 Tonnes of vertical load. In most heavy-duty equipments, maximum loads of 5-6 Tonnes are present, meaning that the load bearing is not an issue at all. Furthermore, the coefficient of friction of PTFE against another surface only reduces with the application of pressure – implying that apart from taking the load, the effectiveness of the wear plate in ensuring a smooth functioning of parts is greatly enhances.
But much like people can look at electronics in google and get completely the wrong end of the stick, I'm hoping someone will tell me why, or why not PTFE is good ?
I'm still none the wiser.
Dave
And then "PTFE polymer will cold flow at pressures of 2,000 psi which limits the polymers usefulness as a bearing material" which is a very high pressure !
Then Top 5 Materials for Plastic Bearings Used on Metal Surfaces
And : 2.Load bearing
A common misconception relating to the load bearing capacity of PTFE leads many machine tool builders to write-off PTFE as a wear pad material. The assumption is that phosphor-bronze, being a metallic material, is the only option strong enough to take the load of heavy moving parts.
In truth – PTFE has a compressive strength of at least 135-140Kg per square cm. This implies that a 100mm x 100mm plate would be able to withstand 13.5-14 Tonnes of vertical load. In most heavy-duty equipments, maximum loads of 5-6 Tonnes are present, meaning that the load bearing is not an issue at all. Furthermore, the coefficient of friction of PTFE against another surface only reduces with the application of pressure – implying that apart from taking the load, the effectiveness of the wear plate in ensuring a smooth functioning of parts is greatly enhances.
But much like people can look at electronics in google and get completely the wrong end of the stick, I'm hoping someone will tell me why, or why not PTFE is good ?
I'm still none the wiser.
Dave
Probably best to Google it as I'm certainly no expert, but PTFE is not mechanically stable and will flow with pressure on it. Perhaps Stuart will chime in with the proper explanation and assessment of its suitability for this purpose.
The ball should have an interference fit in the bushing and not rotate. The ball being hardened and the cap being very soft, it'd grind through the cap in very short order if it were allowed to rotate. While many people believe it does or can rotate due to the dimple witness mark on the inside of the cap, I don't believe it does. It's certainly not intended to, and if it were there'd be ground metal deposits in the cap which I've never seen.
Here are my thoughts on the matter: http://www.diyaudio.com/forums/anal...ics-sp-10-mk2a-project-6l6-7.html#post4064326
The ball should have an interference fit in the bushing and not rotate. The ball being hardened and the cap being very soft, it'd grind through the cap in very short order if it were allowed to rotate. While many people believe it does or can rotate due to the dimple witness mark on the inside of the cap, I don't believe it does. It's certainly not intended to, and if it were there'd be ground metal deposits in the cap which I've never seen.
Here are my thoughts on the matter: http://www.diyaudio.com/forums/anal...ics-sp-10-mk2a-project-6l6-7.html#post4064326
jp, I'm still wondering why you said this ??? It seems pretty definitive to me, a sort of statement of fact from you ?
Did you read my links, can you send me some links too please ?
Dave
Dave - to the best of my knowledge it's not disputed that it does cold flow. It not being suitable as a thrust pad material is my opinion only, hence the IMO. I've no training or education in polymers - my thoughts are based on what I've read and subjective experimentation.
Personally it's not something I'd do unless a trusted expert said it'd work fine in that application, especially when the consequences are considered.
Personally it's not something I'd do unless a trusted expert said it'd work fine in that application, especially when the consequences are considered.
Ok guys, I am not Stuart, but I think these should give some ideas:
Wiki:Creep (deformation) - Wikipedia, the free encyclopedia
Creep of polymers section:
"Viscoelastic creep data can be presented in one of two ways. Total strain can be plotted as a function of time for a given temperature or temperatures. Below a critical value of applied stress, a material may exhibit linear viscoelasticity. Above this critical stress, the creep rate grows disproportionately faster. The second way of graphically presenting viscoelastic creep in a material is by plotting the creep modulus (constant applied stress divided by total strain at a particular time) as a function of time.[2] Below its critical stress, the viscoelastic creep modulus is independent of stress applied. A family of curves describing strain versus time response to various applied stress may be represented by a single viscoelastic creep modulus versus time curve if the applied stresses are below the material's critical stress value."
And now we go to PTFE hand book http://www.rjchase.com/ptfe_handbook.pdf
Page 16 and further.
So I can say that for our application PTFE should be good enough.
Wiki:Creep (deformation) - Wikipedia, the free encyclopedia
Creep of polymers section:
"Viscoelastic creep data can be presented in one of two ways. Total strain can be plotted as a function of time for a given temperature or temperatures. Below a critical value of applied stress, a material may exhibit linear viscoelasticity. Above this critical stress, the creep rate grows disproportionately faster. The second way of graphically presenting viscoelastic creep in a material is by plotting the creep modulus (constant applied stress divided by total strain at a particular time) as a function of time.[2] Below its critical stress, the viscoelastic creep modulus is independent of stress applied. A family of curves describing strain versus time response to various applied stress may be represented by a single viscoelastic creep modulus versus time curve if the applied stresses are below the material's critical stress value."
And now we go to PTFE hand book http://www.rjchase.com/ptfe_handbook.pdf
Page 16 and further.
So I can say that for our application PTFE should be good enough.