Resistance from a drag washer, re: average diameter regardless of surface area

[jurelometer]

Hey Jason:

The original guy could have gotten more drag because the new material was thicker and he could tighten the drag more.  Or it could have been fresh CF with a higher CoF.  Lots of stuff to look at before discounting the basic friction formula.

Regarding stacking washers:  if the drag surface pairs in combination are independent of other drag surface pairs that share a common clamping load, then the effect is additive.  This one stumped me at first too.

Regarding testing:  measuring drag is not enough. You also have to measure clamping load.  Measuring the torque on the star could be a rough stand-in unless the amount of lubricant on the threads changes a bit.  Then the ratio of torque to clamping load changes, and your estimate is going to be even farther off.

And lots of other things to look at like a common break-in period on the fresh drag, or even better measuring the CoF before testing.

Getting a result that you can have faith in it is actually pretty hard.

-J

[boon]

If we want to get into exact numbers, then I agree it gets  more complicated, but we are not yet in a discussion about exact numbers.  We are discussing simpler stuff, and trying to decide if we can accept the basic law of sliding friction because it seems so counter-intuitive.

That's kinda my point. There is probably very little point in trying to apply the very basic "laws" of friction because they are only true for a very "pure" setup, which the drag in a reel is not. Starting with first principles of friction is useful for a theoretical exercise, but not a practical one.

[jurelometer]

If we want to get into exact numbers, then I agree it gets  more complicated, but we are not yet in a discussion about exact numbers.  We are discussing simpler stuff, and trying to decide if we can accept the basic law of sliding friction because it seems so counter-intuitive.

That's kinda my point. There is probably very little point in trying to apply the very basic "laws" of friction because they are only true for a very "pure" setup, which the drag in a reel is not. Starting with first principles of friction is useful for a theoretical exercise, but not a practical one.

You left out this preceding part:

The basic law of sliding friction  is useful for a basic understanding, plus it is where you need to start.  The engineers that design race car brakes care about all sorts of fancy stuff around friction, presumably with a focus on very large changes in heat. But they still use a frictional surface concentrated on the perimeter of a large disk because the basic law of friction is still in play.

My point is that the basics of friction don't just go away, because the reality is more complicated.  They still teach the basics for a reason.

We are looking at variance in kinetic energy transferred to heat from changing inner and outer diameters of dry sliding flat rotational friction surfaces with the same clamping load and coefficient of friction.  But not down to any significant precision, just as general working knowledge.

And it seems to me that a single dry drag disk/washer pair  on a reel is a fairly simple case if we are only trying to answer these basic questions. But that is maybe because I am only aware of the simpler parts of the physics involved.

If you can enlighten me/us beyond a general statement that this is not a practical approach, I am all ears.


-J

[oldmanjoe]

  The way I look at it , is there is no constant drag number in the stack on a working fishing reel .

   The working diameter of the spool is changing and the gear ratio of the reel weather it be 2 to 1 or 4 to one or even higher ratios change how the drag works at giving spool diameters .       I should say this is before the dog kicks in to play .

I think lever drags maybe more forgiving ..

[Rocket Dog]

 

[jurelometer]

The way I look at it , is there is no constant drag number in the stack on a working fishing reel .

  The working diameter of the spool is changing and the gear ratio of the reel weather it be 2 to 1 or 4 to one or even higher ratios change how the drag works at giving spool diameters .      I should say this is before the dog kicks in to play .

I think lever drags maybe more forgiving ..

I agree with your main point that as the line goes in and out, the spool diameter change  causes the drag to change, so we shouldn't worry too much about minor measurement accuracy.  I would also add that the line being pulled through the water, especially crosswise, can also add significant friction.

But is also still useful to understand the tradeoffs.  For example, a larger OD/ID disk will give you more drag for the same axial load on the system, but will require more rigidity to maintain alignment, and so on...

I would word the star drag statement slightly differently- the gear ratio does act as a divisor: you get only 1/4 of the drag stack torque at the spool on a 4:1 reel.  The ratio remains constant as the spool diameter changes.

Lever drags don't have this divisor, and don't have to load up the gears if designed properly (hint: lever drags with roller clutches on the handle shaft are not "proper"), but also have an axial load issue that has to be managed. 

On a star drag, the undergear thrust washer can have fairly high friction as long as it lays flat and doesn't wear out fast.  Alan often prefers to use an actual drag washer for a thrust washer.  So usually not as much  axial load worry on a star drag stack.  But minimizing axial load required for a given drag setting still has value here.

-J

[boon]

If we want to get into exact numbers, then I agree it gets  more complicated, but we are not yet in a discussion about exact numbers.  We are discussing simpler stuff, and trying to decide if we can accept the basic law of sliding friction because it seems so counter-intuitive.

That's kinda my point. There is probably very little point in trying to apply the very basic "laws" of friction because they are only true for a very "pure" setup, which the drag in a reel is not. Starting with first principles of friction is useful for a theoretical exercise, but not a practical one.

You left out this preceding part:

The basic law of sliding friction  is useful for a basic understanding, plus it is where you need to start.  The engineers that design race car brakes care about all sorts of fancy stuff around friction, presumably with a focus on very large changes in heat. But they still use a frictional surface concentrated on the perimeter of a large disk because the basic law of friction is still in play.

My point is that the basics of friction don't just go away, because the reality is more complicated.  They still teach the basics for a reason.

We are looking at variance in kinetic energy transferred to heat from changing inner and outer diameters of dry sliding flat rotational friction surfaces with the same clamping load and coefficient of friction.  But not down to any significant precision, just as general working knowledge.

And it seems to me that a single dry drag disk/washer pair  on a reel is a fairly simple case if we are only trying to answer these basic questions. But that is maybe because I am only aware of the simpler parts of the physics involved.

If you can enlighten me/us beyond a general statement that this is not a practical approach, I am all ears.


-J

https://www.sciencedirect.com/topics/engineering/amontons-law

Amonton's second law only holds true when there is no adhesion. This is not the case in virtually all real-world applications, therefore the notion that surface area does not affect total friction is incorrect, rendering pointless all further theories that rely upon that being true.
It is not a practical approach because it relies upon a "law" that is simply not true for the application being considered, and leads us to incorrectly discount a completely valid premise, which is that increasing the surface area of the drag increases the total force required to overcome it (to some degree, however small). Other factors which you have highlighted likely contribute to the increase in overall drag force, but in light of the above, the statement below is not entirely applicable, no?

Sliding friction is not a function of surface area.  so it doesn't matter if you add more surface area or how many pieces the surface area comes in. 

[Vogelspinnen]

So I've been reading up on here and on the interwebs, and while I'm nowhere as 'sciency' as you guys here, I can mostly understand what you guys are talking about, and would like to give my 2 cents in layman terms.

So it seems that J is spot-on about how surface area does not affect friction, I think why it seems counterintuitive to us is for 2 reasons:

1. More surface area will definitely give more friction if PSI is the same. But since we are applying the same pressure with the star drag, the pressure is distributed across a wider surface area, effectively reducing PSI.

2. My thoughts on race car tires having more grip with slicks and fatter tires does not hold water, because there are other forces at work in a race car tire. The soft tire actually holds onto the small imperfections of the road much like a Velcro effect, which is why surface area matters, because more surface area = more 'adhesion'. In a fishing reel our washer surfaces are mostly smooth, which reduces this effect significantly. If we were to have a metal washer that is grooved surface area will definitely affect drag power, but our drag washers wouldn't last very long.

3. The added drag I am seeing could be due to a variety of reasons, from inconsistent torquing down of the star drag, to the new washer being slightly thicker and newer, and definitely dryer (I have never been able to completely degrease the old one).

In any case, in the name of science and having more drag to stop those angry snakeheads, I will be getting a new drag washer to replace the outer one as well, and run it dry this time. I will report with the results!

[JasonGotaProblem]

I love science. I truly do. Our collective understanding has increased. And that's the victory.

[Gfish]

Vogelspinnen, maybe you can mark the star position with white-out or something in-between tests of original washer vs 2nd washer modification—-max. drag. One mark on reel body, one on the star point. Still not an exact science type of thing, but a 2lb. difference in drag friction is significant.

[oldmanjoe]

So I've been reading up on here and on the interwebs, and while I'm nowhere as 'sciency' as you guys here, I can mostly understand what you guys are talking about, and would like to give my 2 cents in layman terms.

So it seems that J is spot-on about how surface area does not affect friction, I think why it seems counterintuitive to us is for 2 reasons:

1. More surface area will definitely give more friction if PSI is the same. But since we are applying the same pressure with the star drag, the pressure is distributed across a wider surface area, effectively reducing PSI.

2. My thoughts on race car tires having more grip with slicks and fatter tires does not hold water, because there are other forces at work in a race car tire. The soft tire actually holds onto the small imperfections of the road much like a Velcro effect, which is why surface area matters, because more surface area = more 'adhesion'. In a fishing reel our washer surfaces are mostly smooth, which reduces this effect significantly. If we were to have a metal washer that is grooved surface area will definitely affect drag power, but our drag washers wouldn't last very long.

3. The added drag I am seeing could be due to a variety of reasons, from inconsistent torquing down of the star drag, to the new washer being slightly thicker and newer, and definitely dryer (I have never been able to completely degrease the old one).

In any case, in the name of science and having more drag to stop those angry snakeheads, I will be getting a new drag washer to replace the outer one as well, and run it dry this time. I will report with the results!

Are you cutting your own washers , do you have other type of material to work with ?

[jurelometer]

https://www.sciencedirect.com/topics/engineering/amontons-law

Amonton's second law only holds true when there is no adhesion. This is not the case in virtually all real-world applications, therefore the notion that surface area does not affect total friction is incorrect, rendering pointless all further theories that rely upon that being true.
It is not a practical approach because it relies upon a "law" that is simply not true for the application being considered, and leads us to incorrectly discount a completely valid premise, which is that increasing the surface area of the drag increases the total force required to overcome it (to some degree, however small). Other factors which you have highlighted likely contribute to the increase in overall drag force, but in light of the above, the statement below is not entirely applicable, no?

Sliding friction is not a function of surface area.  so it doesn't matter if you add more surface area or how many pieces the surface area comes in. 

Thanks for the link.

I would first like to point out a for the record that while I am getting nicked on this thread for oversimplification (rightfully, I might add), I am getting nicked on the thread that spawned this one for potentially scaring folks off for being too technical. On the exact same topic.  Can't win

I would also like to note that we are probably better off not just focusing on how much (or little) surface area matters. I don't want to leave behind the discussion  on the importance of where that surface area resides (how much of it is farther from the disk center).

Ok, now getting back to the question of the surface area effect dry sliding friction:

I do agree that it was incorrect to say that surface are does not matter at all. I should have said that it does not matter much for simpler cases like ours.   For the folks playing spectator:

Relatively recent advances in the science of tribology have turned this topic into a complex one, but starting way back in the 1500s or so, sliding friction was considered  the result of load and mechanical resistance. The surfaces resisted sliding because they had ridges, pits, grooves, etc.  Even if the surface appeared smooth, the imperfections were there, however tiny. It was thought that friction was simply the force required to overcome these mechanical barriers to movement.  Some basic experiments can be be performed  that support this.  You can predict the amount of force required if you know only the force pressing the surfaces together and the coefficient of friction for the surface pair, and surface area does not matter.  But as Boon pointed out, these basic laws break down as the situation becomes complex. 

One reason behind this is that this resistance to sliding is not just mechanical.  there are molecular forces in play, such as inter-molecular attraction between some materials of the different surfaces.  There can still be some friction when the surfaces are smooth down to the molecular level. This is what is being referred to as adhesion in friction (at least from this layman's viewpoint).

More surface area, more molecules potentially in contact, and therefore the greater potential for adhesion.  But the contribution of adhesion to total friction will vary.

There will be cases when the basic formula will be reasonably accurate, which means that surface area will not have an significant effect of friction, and cases where the formula starts breaking down.

So the debate sort of boils down to  this: Is the case of a single carbon fiber washer spinning against a lightly polished stainless steel disk at fairly high loads with low to moderate speeds in an unsealed environment within the bounds of the basic laws of friction, or is this a situation  where adhesion plays enough of a role that the basic formula breaks down and surface area also comes into play?

It should be noted that while adhesion is always present to some degree, the coefficient of friction includes the effect of adhesion.  So I am not convinced by the argument that the simple existence of adhesion makes the basic laws of friction nearly useless.  When  adhesion is not a dominant factor in the coefficient of friction, then surface area should not have a significant effect on friction.

I am thinking that when looking at simpler situations in a simple fashion, we don't have to worry too much about adhesion.

There are papers that support this viewpoint, for example:

The importance of adhesion between slider and support in frictional phenomena is reviewed. The conclusion is that it is negligible as long as the term friction is used in its common sense, i.e. as long as frictional force is reproducible for a given path and is a definite function of the normal load. The basic reason for the absence of adhesion in air is that both slider and support are covered with adsorbed films (which represent a weak boundary layer) so that no atomic contact between the two solids is present.

https://www.sciencedirect.com/science/article/abs/pii/0043164876902180

Also note that even the coefficient of friction is not correctly a single value. It will vary depending on some of the factors mentioned above, and probably some other stuff.  Nevertheless, the simple single-value  coefficient of friction is listed on material tables everywhere and has value even if it can not always be applied directly in more complex situations.

When designing advanced machine tooling, all the friction details matter. For estimating the drag on a simple baitcaster, I am still at meh.

For now, I am sticking to my guns on surface area not mattering (much), but admit that I should have been less absolute in my original statement.

-J

[boon]

I know the plural of anecdote is not data but anecdotally it seems to be the case that increasing the surface area of a reel's drag commonly leads to a (highly non-scientific, heh) perceived increase in drag.

More broadly, most of the time in the context of AT.com it's "I did some mods to my reel and I got more drag". There's not enough control (nor really the intent, to be honest) to determine with any particular robustness the exact reason for the increase - it's good enough that it achieves that goal, often spectacularly.

In the general scope of "mods to improve a reel's performance" there are other reasons for larger surface area, too - more surface area is going to conduct heat better out of the drag washer into whatever the "sink" is (the spool on a lever drag or the drive gear, shaft, etc on a star drag) so there are other benefits to doing it.

With my cynical hat on, I wouldn't exclude the possibility that reel manufacturers have discovered they can cut drag washers for multiple reels in concentric rings and thus get more drags from a sheet of CF, or something equally driven by the goal of reducing manufacturing cost while still achieving the required level of performance from the reel. Beancounters over engineers

Summation of my thoughts: Does drag surface area increase drag force? Probably. A meaningful amount? Dunno 

[oldmanjoe]

   All good points to look at . I hope people do take the time to read the papers posted .  I need to catch up on the reading myself .  Understanding each component of the drags is key .

  This thread can break down all the things that can be done to build a drag stack for different              applications .  We can build stacks for low drag numbers or pull the anchor up and everything in between .

   A instrument can make music , but it takes a few to make a concert . 

[JasonGotaProblem]

This thread has been great.

Dave, I appreciate the effort you put into your posts. And I appreciate that someone takes on the sometimes tedious duty of reminding us that the laws of physics don't care about our ideology or whether we understand them. We all need a "mr buzzkill" in our lives to keep us in line.

But I'm also glad I followed my instincts when it seemed like an overly reductive interpretation lost some relevant nuance. I am not burdened by the notion that I understand any of this fully. In fact it's quite liberating to accept that I'm not an expert. I don't think I actually made any useful statements. I just asked annoying questions.