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Wear of aluminum alloys

06/02/2007 10:59 AM

Hi,

It's my turn to put a question. I am confronted with a plane sliding bearing where the static surface will be an aluminum alloy and the moving element a brass piece.

The actual solution matches the brass piece with a steel surface and there are no problems for about 100.000 cycles, after those cycles the surfaces are still as new, but for cost and weight reasons I would like to go aluminum.

I would like to know if anybody knows how -quantitavely- the wear depends on exercised pressure. It is possible to make a hard anodized surface but of course it will be better to avoid this cost. The stroke is very small 0.5...1 mm and the surfaces can be lubricated with a graphite grease. Sliding speed is very low and frequency 0.1 Hz or even less. Over the stroke the pressure goes from nil to max in the direct stroke and commes again to zero in the reverse stroke.

I would highly appreciate if somebody has experimental data or could give a hint where I can find some values for wear and if possible for the friction coefficient.

Thank you guys in advance,

Nickname

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#1

Re: Wear of aluminum alloys

06/03/2007 7:03 AM

The cof of aluminum is about 3x's that of steel. Could be trouble.

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#2
In reply to #1

Re: Wear of aluminum alloys

06/03/2007 7:10 AM

Sorry but i do not understand. as far as I know wear is proportional to pressure (among other factors) so that in reducing pressure there is a possibility to avoid for a limited number of cycles a too important wear. Do you know any values I could use as references? By the way what do you mean by the cof? coefficient of friction? in which situation : dry or lubricated? and how big is it for aluminum (steel varies between 0.1 and 0.25)? Did you met anywhere an information about wear rates (mm^3/m at a given pressure)? I shall be very thankful for any even partial answer. I ask because I am aware that it can be trouble if limits are not considered.

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#3

Re: Wear of aluminum alloys

06/03/2007 11:51 AM

You may find the data here useful.

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#4

Re: Wear of aluminum alloys

06/03/2007 12:25 PM

Hi nickname,

Found lots of tables with good data but can't seem to find an aluminum/brass combination.

http://www.roymech.co.uk/Useful_Tables/Tribology/co_of_frict.htm has lots of good data. Hope it helps.

John

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#5

Re: Wear of aluminum alloys

06/03/2007 4:44 PM

Hi,

forget about sliding aluminum without anodising - no other surface will give good results as the aluminum is much too soft.

If anodising go to hard anodising its much better for your purpose.

AlSi-alloys are good also AlMgSi for anodising, AlCu and AlZn are more difficult.

Forget about Pb containig alloys.

Do not let the anodiser seal the surface but do impregnate this with the lubricant you have in mind.

What about anodised to anodised surface in contact, this needs no lubricant if wetted by water. Brass is more expensive than Al.

Did you think about a flexure mechanism to replace your sliding mechanism?

With your small stroke this schould be possible.

I did design many different flexure mechanisms (simple to highest accuracy, any temperature operation, many different materials, very different temperatures) since a long time - but this will take some time and money.

regards

RHABE

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#6
In reply to #5

Re: Wear of aluminum alloys

06/03/2007 5:36 PM

Thank you very much, of course I am aware of the problems. I cannot avoid the sliding-unfortunately- so that i have to live with it i shall try to reduce as much as possible the contact pressure and if no other way to invest in anodizing.

Does any body know how is wear rate on anodized surfaces in mm^3/mm stroke at a given pressure? I would like to estimate -roughly- the number of cycles. The other problem is that the superficial layer of AlO3 is brittle so that if a too big pressure is applied it can crack and the hard particles will further destroy the contact. Does any body know pressure allowable values for anodized surfaces?

It is great to get such attention and help, thanks to all of you

Nickname

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Anonymous Poster
#7
In reply to #6

Re: Wear of aluminum alloys

06/03/2007 9:58 PM

Wear rates are not constant... increase with wear. Visit you local tribology department.

John

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#8
In reply to #7

Re: Wear of aluminum alloys

06/04/2007 1:26 AM

You are right ONLY if wear is so important that the tribological conditions do change. In fact if conditions are within limits wear rate is almost constant for a long period.

Any way I appreciate the counsel i shall try to find my tribology department i do not have because if i would have had one i would not ask for help.

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#9

Re: Wear of aluminum alloys

06/04/2007 12:32 PM

You may also want to consider a graphite impregnated brass bushing/bearing.

Check out:

www.misumi.com

Also, could the brass be replaced with a recirculating ball-bearing?

Just a thought. Good Luck.

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#10

Re: Wear of aluminum alloys

06/04/2007 3:23 PM

Once more:

naked aluminum against anything else will never survive 100000 cycles.!

Anodising is not really expensive.

If you want to have it cheap you can let the main parts without anodising and use two anodised rings as simple bearing.

RHABE

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#11
In reply to #10

Re: Wear of aluminum alloys

06/04/2007 6:31 PM

Hi Rahbe,

I highly appreciate your comments and thank you for them.

Could you please tell me how high i should limit the pressure on a anodized surface? How thick should be the layer? Is it any relationship between pressure limit and thickness?

In fact the goal is to reach 150.000 to 200.000 cycles. It is a limited life application and this number of cycles is for a period of 2 to 3 years.

For all the participants I would like to add following information:

- brass is used since there are other tribo-logical constrains on the part

- it is a flat part which has a gliding movement and this gliding bearing surface cannot be replaced by an elastic hinge or a shaft-bushing pair

Brass is used as a heat conducting material but also as a friction surface on a steel counterpart in order to avoid galling there is already on this other surface made usage of a special solid lubricant.

Of course I did not give from the very start those informations because i only wanted to have a quantified input about wear rate in a friction pair where one of the parts is made of an aluminum alloy anodized or not. From the qualitative point of view I was aware of what was mentioned in the different comments but i did not have a reference value for the criteria to choose the contact area. Some of the link i got helped me and i hope via a similitude with other friction pairs to approximate a wear rate value. I shall use informations about journals and try to relate them to the physical properties of involved materials.

When results will be available shall inform you about.

I thank you all for given support,

Nickname

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#12
In reply to #11

Re: Wear of aluminum alloys

06/07/2007 4:43 AM

pressure on a anodized surface?

Hi,

I am not absolutely shure but I think that the allowable pressure is limited by the oxide that grows at anodising and another limit is the yield strength or the fatigue strength of the al-alloy you select.

brass is used

You definitely do not have to use brass, two anodised surfaces are pretty good for sliding.

heat conducting material

Aluminum has 50% of the heat conductivity of copper, is brass better?

his gliding bearing surface cannot be replaced by an elastic hinge

Any movement can be made by an elastic device if the travel is not too large.

we made rotary, linear, helical, linear to rotary and more

RHABE

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#13
In reply to #12

Re: Wear of aluminum alloys

06/07/2007 10:40 AM

Hi,Rhabe

I appreciate that you invest yourself so much and i feel obliged to give you some more explanations. In a general way your comments are right if you take them from the context which is more complex.

pressure on a anodized surface? there are as you say two limits for the allowable pressure on a limited surface : one is as i mentioned the limit which although deforming the surface will not crack the anodized layer, if qualitatively you mentioned it as well unfortunately you do not give any value or my question was quantitative i expected an answer as for instance: the limit is so much M Pa or ksi depending where you are and which units you use. As you know the deformation is the most dangerous at the border line where the curvature radius is the smallest and where the bending stress is the highest. In compression regions the bending leads to tensions and the risk of cracks is real.

The other limit is the under layer fatigue which has a higher importance if the shear stress is high. On a lubricated surface this shear has a low influence on the von Mises resulting stress so that the fatigue will not be affected very much. The contact pressure is the main limiting parameter. This is valid for this contact surface.

There is of course a 3rd limit given by the capability of the parts to eliminate the generated heat. The diffusivity of ALU alloys or cooper alloys does not differ too much so that from this point of view both could be accepted.

brass is used

heat conducting material

The same piece has a second function and works as a sliding brake. The counter part for different reasons has to be made of a high resistance steel. In such a pair one of the parts has to be weaker and also lead the generated heat if it is the non moving one. This surface works with a lot higher shear strains and has to be as well the soft surface. In higher strains i would not dare use ALU since there are some risks, i cannot anodize it since it has to stay soft. This was the reason to choose brass because it is as well available in sheets and not only in bars. In this application a cooper alloy would offer more advantages than an ALU alloy. There was also a cost reason : the steel surface due to its manufacturing cost has to be the low wear component.

his gliding bearing surface cannot be replaced by an elastic hinge

The problem of hinges is that either they are stiff or they ask for a lot of space. In fact I am VERY accustomed with "hinges" since i work in force and torque transducers and such designs are often used as mechanical uncoupling elements. But it is not always the optimal solution. As you know a solution is to be accepted not only because it is "nice" but also if it is economical. And for this application the hinge was and is still not a solution for the problem.

For your information since i could not get values i build up a model and analyzed (fea) the al2o3 layer deformation on an ALU support. I know how ceramic (same composition) behaves and i determined at which pressure for a given edge radius the layer will reach the limit for ceramics. I know that i take a risk but since nobody was in the position to give a quantitative answer i had to find a solution. Any way I had the intention to go directly to a prototype based on experimental values now i shall first go to a research model and the prototype will come later. For the first lot the material will be a low resistance steel.

Thank you again

Nickname

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#14
In reply to #13

Re: Wear of aluminum alloys

06/07/2007 4:59 PM

Hi nickname,

if you take 50% of the yield strength of the al-alloy you choose you are at a good estimate, if you take 30% you are on the safe side.

If you need low strength take Al6061 (easy to anodise), moderate strength Al2024 high strength: Al7075.

Temper this to T73 or T76.

Let it hard anodised without sealing. (Many anodisers will not like the 2024 and the 7075)

Design your parts so that elastic deformation is not deteriorating the surface flatness required for match of the two surfaces.

Lubricate the surface with your lubricant as soon as you get it without touching the surface with fingers or other stuff.

Make a test with the required number of strokes.

Good luck

RHABE

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Anonymous Poster (2); GM1964 (1); Johnjohn (1); nick name (5); RHABE (4); unclefastguy (1)

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