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#246 - 08/05/05 11:50 AM Intensity too high - fixed wheel speed
Dr Peener Offline


Registered: 01/05/05
Posts: 35
Loc: Mishawaka Indiana, USA
[From an e-mail sent to Dr. Peener]

I need to resolve some doubts. Could you help me?

We produce parabolic leaf springs and we have determined through many years and by many fatigue tests that with an arc height of the Almen C strips of 0,20 0,04 mm, the results in fatigue of our springs are good and fulfill with our customers expectations, obtaining about 1200 Mpa of compressive residual stresses at 0,15 mm of depth.

To achieve this, we use a 4 turbines machine (37 KW, 380V, 2500 rpm, 500/135 diameter , 8 blades with a normal intensity of 75/25 Amp. full/empty.
This gives in theory a shot speed of 87 m/seg that I can't change because 2500 rpm is an immovable value.
The shots we are using are 1,19 mm and they are automatically removed when arrive to 0,60 mm.
The conveyor velocity is normally 4 m/ min.
The coverage on the leaves it seems very good, perhaps 98 %???

But when we make saturation curves with 1,2,4 and 8 times of exposure and following the 10% rule, the Almen C intensity obtained is about 0,35 mm which is very high and probably not very good for the surface of the spring. Of course we only pass 1 time the springs through the machine and in these conditions we are obtaining on the strips 0,20 0,04 mm.


Questions:

If 0,35 mm is the real Almen C intensity of the process ( in saturation) and perhaps 0,20 mm is only the arc height of the Almen C strips, what is the intensity on the leaf springs?.


If we don't saturate the leaf springs, the process is not correct?


If the springs were passed through the machine 4 times in order to saturate or we reduce the conveyor velocity until perhaps 1m/min ( of course this is absolutely impossible under the productivity point of view) what would be the benefit in terms of quality? Excessive coverage may create surface damage, where is the equilibrium?


Are we wasting shots and reducing life of machine components by using extremely high Almen intensity without necessity ?.


How to do to reduce the Almen C intensity of the process from 0,35 to 0,24 or maximum 0,26 if we can't reduce rpm of turbines neither shot characteristics ( hardness, size, etc) ?. Reducing the shot flow could affect negatively to coverage, perhaps reducing shot flow and conveyor velocity as much as possible ?.

Your only good option is you could reduce the size of the shots. The smaller shot size has less energy (still at 87m/sec) and therefore the peening intensity will be reduced. However, you will have many more shots/kg of media and therefore you will be creating many more dents/minute. You should then experiment with conveyor velocity and shot flow rate. You might want to run the conveyor at its highest speed (highest production rate) and then experiment with shot flow rate. You want the lowest flow rate that just gives you 100% coverage at the high conveyor speed. You might be surprised that the shot flow rate could be quite low. You'll have to experiment.


Where is the mistake, perhaps in the design of the turbines (rpm should be easily controlled) or the shot size is too big and needs a lot of denting time ?

Thank you for your help
_________________________
Jack Champaigne (Dr. Peener)
Editor The Shot Peener

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#247 - 08/05/05 12:01 PM Re: Intensity too high - fixed wheel speed
Jack Champaigne Online   content


Registered: 04/20/99
Posts: 319
Loc: Mishawaka, IN, USA
[I'll respond with --> ]
Questions:

If 0,35 mm is the real Almen C intensity of the process ( in saturation) and perhaps 0,20 mm is only the arc height of the Almen C strips, what is the intensity on the leaf springs?.

---> You are correct, 0,35mm is the intensity of the process and 0,20 mm is only an arc height reading for a single pass through the machine. The intensity of the leaf springs is not a good term. The intensity of the shot impacts on the leaf spring are 0,35 mm. This is true for a single impact or sufficient impacts to completely cover the leaf spring, such as 98% or 100% coverage.

If we don't saturate the leaf springs, the process is not correct?

---> The word saturate may cause confusion. To saturate the leaf spring should mean it is completely dented with shot impacts. This is verified by visual inspection (usually 10 power magnification). I presume you achieve complete denting of the leaf spring in a single pass through the machine. This allows enough exposure time to accumulate dents to sufficiently cover the entire surface. This is based upon exposure time and shot flow rate.


If the springs were passed through the machine 4 times in order to saturate or we reduce the conveyor velocity until perhaps 1m/min ( of course this is absolutely impossible under the productivity point of view) what would be the benefit in terms of quality? Excessive coverage may create surface damage, where is the equilibrium?

---> You don't need to pass the leaf springs through the machine 4 times in order to saturate. Passing them through once provides complete coverage. However, you may have to pass the Almen strip through 4 times in order for the saturation curve to exhibit the 10% rule.

Are we wasting shots and reducing life of machine components by using extremely high Almen intensity without necessity ?.

---> YES

How to do to reduce the Almen C intensity of the process from 0,35 to 0,24 or maximum 0,26 if we can't reduce rpm of turbines neither shot characteristics ( hardness, size, etc) ?. Reducing the shot flow could affect negatively to coverage, perhaps reducing shot flow and conveyor velocity as much as possible ?.

---> Well, obviously, if the machine design had variable speed turbines you would be adjusting the shot velocity and hence intensity. Unfortunately variable speed drives are expensive and not often used. Since your machine did have a fixed speed more care should have been exercised to determine intensity with Almen strips and a good saturation curve. You must also have a clear understanding that you first set the machine and shot parameters to attain the requested intensity. That's phase one. The next phase is to determine the appropriate conveyor speed and shot flow rate so that you achieve complete, 100% denting, with only one pass through the machine.


Where is the mistake, perhaps in the design of the turbines ( rpm should be easily controlled) or the shot size is too big and needs a lot of denting time ?

Your only good option is you could reduce the size of the shots. The smaller shot size has less energy (still at 87m/sec) and therefore the peening intensity will be reduced. However, you will have many more shots/kg of media and therefore you will be creating many more dents/minute. You should then experiment with conveyor velocity and shot flow rate. You might want to run the conveyor at its highest speed (highest production rate) and then experiment with shot flow rate. You want the lowest flow rate that just gives you 100% coverage at the high conveyor speed. You might be surprised that the shot flow rate could be quite low. You'll have to experiment.

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#248 - 10/17/05 03:12 PM Re: Intensity too high - fixed wheel speed
R. Stone Offline
Member

Registered: 10/17/05
Posts: 2
Loc: Indiana
I have run across a similar situation in the past. In that case, the problem was corrected by replacing the blast wheel blades with shorter blades. This reduced the shot velocity without changing the wheel speed. Are your turbines direct-drive or belt-drive. If they are belt driven, you can also affect the shot velocity by selecting different shieve sets.

Good luck,

RGS

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