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My experiment...

E

Eldon (WA)

Well-known Member
looks like a success. My newer bush hog 3008 wasn't cutting quite as nice as I would have liked. I went to change the blades and found out they now use metric nuts. I don't have a big set of metric sockets, so I decided to just crawl under it with the 7" angle grinder. I held the grinder at around 75 degrees and cut notches about every half inch to make them serrated. Just did 3 hours cutting knapweed and it worked really nice. Guess what I am going to do with the 5 old sets of blades I have laying out back...not sure what to do with the 3 sets of new blades sitting in the barn.
 
Let me guess, for the old blades you are going to:

1) notch them, and then
2) buy at least one large metric socket

How'd I do ????
 
(quoted from post at 17:57:12 07/22/18) Let me guess, for the old blades you are going to:

1) notch them, and then
2) buy at least one large metric socket

How'd I do ????
Click to expand...

#1, yes, #2 I may just borrow a socket from a friend and then grind the nut down to fit my regular socket. I remember that is what I did when I replaced the nuts on the old one a couple years ago.
 
(quoted from post at 19:06:02 07/22/18) Mine get serrated from all the rocks I hit!
Click to expand...

The ones on my old mower that I use most of the time developed serrations after about 300 hours and I noticed they were still cutting nice, that is why I had this idea. They currently have close to 600 hours on them and still cutting well. I hit a lot of ground squirrel mounds in gravelly soil, I'm sure that is what started the wear pattern on them. By using the grinder at that angle I can sharpen the serrations a lot quicker than trying to remove enough material to give a factory edge. As long as I don't hit a lot of big rocks and round the ends of the blade off they should be good to go.
 
Might give that a try. You cut the angle away from the hub, or toward the hub? Might be on to something. I have never seen a notched blade on mower blades. Stan
 
(quoted from post at 19:50:01 07/22/18) Might give that a try. You cut the angle away from the hub, or toward the hub? Might be on to something. I have never seen a notched blade on mower blades. Stan
Click to expand...

It was toward the hub...the only comfortable way I could hold the grinder. I don't think it really mattered as long as it was notched.
 
Yeah but ? that sort of notch can cause fatigue failure in a rotating vibrating blade. Buddy of mine had a bush-hog blade fail, snap off a 6" piece, and he heard it fly by his head, missed him about a foot or two. He happened to see it as it went flying past, drove over and retrieved it. Fatigue failure is a sudden, catastrophic failure, caused by repeated high back-and-forth stresses in a cracked part.
 
(quoted from post at 05:50:33 07/23/18) Yeah but ? that sort of notch can cause fatigue failure in a rotating vibrating blade. Buddy of mine had a bush-hog blade fail, snap off a 6" piece, and he heard it fly by his head, missed him about a foot or two. He happened to see it as it went flying past, drove over and retrieved it. Fatigue failure is a sudden, catastrophic failure, caused by repeated high back-and-forth stresses in a cracked part.
Click to expand...

I'd like to see the stress analysis on how a 1/8" notch could cause a fatigue failure on a blade that is 1/2" thick and 4" wide....your buddy must have been talking about his lawn mower :)
 
(quoted from post at 17:54:45 07/23/18) Picture?
Click to expand...

20311.jpg
20312.jpg
20313.jpg
 
An 1/8" notch is a large initial crack when it comes to fatigue. With a presume hardened blade that could spell disaster. The things that affect fatigue life:
1. Size of the initial crack (in this case 1/8") The larger the initial crack the less "Life" it has. At some point, the crack length will go from a ductile (slow growth) to brittle fatigue (think glass)
2. The type of material, usually the higher the strength (or in this case hardened) the lower the "toughness" and the quicker a crack will grow (there is less plasticity at the crack tip so it grows faster)
3. The stress range (note this is not absolute stress). The fatigue will be worse if the nominal stress is only 30% of failure, but the stress range is 20% of yield, than if it is 70% of yield with only 10% stress range. (Sr)
4. The number of cycles (Nc) (This can be caused by vibration (high number of cycles) or by impact (low number or cycles)
5. The Stress Concentration Factor (SCF) (the peak stress near the crack divided by the nominal stress). This probably is somewhere around 3 to 5 for your blades.
6. The material exponent on the Stress Range. For steel it is usually in the range of 3-5.
7. The total number of cycles expected at a given stress range (Tc) which is depended on the initial crack size

The total life (as a percentage) at a given stress range = (SCF * Sr) ^ 3 * Nc / Tc (If you set this equation equal to 1 then you can back out the EXPECTED number of cycles at a given stress range until failure.)

Notice the exponential effect of the stress range (and SCF) when it comes to fatigue.

My recommendation would be to change the blades so they don't fail catastrophically.

bjb
Texas Structural Engineer

PS In some specialty high strength metals, the critical crack (before it becomes brittle and fails) is only thousandths of an inch.
 
(quoted from post at 05:47:05 07/24/18) An 1/8" notch is a large initial crack when it comes to fatigue. With a presume hardened blade that could spell disaster. The things that affect fatigue life:
1. Size of the initial crack (in this case 1/8") The larger the initial crack the less "Life" it has. At some point, the crack length will go from a ductile (slow growth) to brittle fatigue (think glass)
2. The type of material, usually the higher the strength (or in this case hardened) the lower the "toughness" and the quicker a crack will grow (there is less plasticity at the crack tip so it grows faster)
3. The stress range (note this is not absolute stress). The fatigue will be worse if the nominal stress is only 30% of failure, but the stress range is 20% of yield, than if it is 70% of yield with only 10% stress range. (Sr)
4. The number of cycles (Nc) (This can be caused by vibration (high number of cycles) or by impact (low number or cycles)
5. The Stress Concentration Factor (SCF) (the peak stress near the crack divided by the nominal stress). This probably is somewhere around 3 to 5 for your blades.
6. The material exponent on the Stress Range. For steel it is usually in the range of 3-5.
7. The total number of cycles expected at a given stress range (Tc) which is depended on the initial crack size

The total life (as a percentage) at a given stress range = (SCF * Sr) ^ 3 * Nc / Tc (If you set this equation equal to 1 then you can back out the EXPECTED number of cycles at a given stress range until failure.)

Notice the exponential effect of the stress range (and SCF) when it comes to fatigue.

My recommendation would be to change the blades so they don't fail catastrophically.

bjb
Texas Structural Engineer

PS In some specialty high strength metals, the critical crack (before it becomes brittle and fails) is only thousandths of an inch.
Click to expand...

Not much of an engineer if you would use a hardened steel blade on a brush hog......
 
(quoted from post at 05:47:05 07/24/18) An 1/8" notch is a large initial crack when it comes to fatigue. With a presume hardened blade that could spell disaster. The things that affect fatigue life:
1. Size of the initial crack (in this case 1/8") The larger the initial crack the less "Life" it has. At some point, the crack length will go from a ductile (slow growth) to brittle fatigue (think glass)
2. The type of material, usually the higher the strength (or in this case hardened) the lower the "toughness" and the quicker a crack will grow (there is less plasticity at the crack tip so it grows faster)
3. The stress range (note this is not absolute stress). The fatigue will be worse if the nominal stress is only 30% of failure, but the stress range is 20% of yield, than if it is 70% of yield with only 10% stress range. (Sr)
4. The number of cycles (Nc) (This can be caused by vibration (high number of cycles) or by impact (low number or cycles)
5. The Stress Concentration Factor (SCF) (the peak stress near the crack divided by the nominal stress). This probably is somewhere around 3 to 5 for your blades.
6. The material exponent on the Stress Range. For steel it is usually in the range of 3-5.
7. The total number of cycles expected at a given stress range (Tc) which is depended on the initial crack size

The total life (as a percentage) at a given stress range = (SCF * Sr) ^ 3 * Nc / Tc (If you set this equation equal to 1 then you can back out the EXPECTED number of cycles at a given stress range until failure.)

Notice the exponential effect of the stress range (and SCF) when it comes to fatigue.

My recommendation would be to change the blades so they don't fail catastrophically.

bjb
Texas Structural Engineer

PS In some specialty high strength metals, the critical crack (before it becomes brittle and fails) is only thousandths of an inch.
Click to expand...

bjb, I am wondering where in your explanation the load comes in. In other words there has to be a load applied measured as bending moment in order to cause the stress. This load would of course vary tremendously depending on how much moment arm there is working on the location where the supposed crack is.
 
The load needs to open the crack tip (in this case the leading edge) which gets developed in three ways.

1. Tension loading radially due to the spinning blades (no moment, but still a fatigue load). The stress range in this case is 0 to full stress at a particular location at the full speed of the blades. The number of stops and
starts is your number of cycles. Note in this setup, tension is not constant; it is zero at the free edge (tips) and maximum near the bolt.

2. Varying load due to the blades hitting air (wind resistance); hitting grass; or in extreme cases hitting say a small tree. This will cause a bending moment on the blades, which also causes tension in the leading edge due
to this moment (note that when the mower is loaded the speed changes the tension as mentioned in part 1 so the stress range due to this mowing "moment" is amplified). In theory, ever blade of grass creates a small
stress range, but you also could look at the speed of the mower due to changes in resistance and create stress range and number of cycles that way as well.

3. Bending and tension caused by vibration due to the out of balanced blades. Here the number of cycles is directly related to the frequency of the vibration.


A few other points:
The way fatigue design works is the previously mentioned S-N curves (Stress verses cycle) curves, plots the maximum number of cycles at any given stress range. Using the previous equation for damage, you can see
how much life as a % is used at any stress range and then add the damage together to estimate the service life.

For most steels there is a minimum stress range to cause fatigue, however, with such large "cracks" in the serration and the expected hardening of the leading edge of the face, makes this a highly likely area for rapid
fatigue damage.

There is a case of "low, cycle fatigue" where at extremely high stress ranges, the life goes from thousands of cycles, to tens of cycles. This would be the case if the mower were to be spinning fully and come to a dead
stop by hitting a large unseen object. A few times will rapidly increase the crack growth.
 
The leading edges of the blade are most likely hardened to some degree, or laminated, so they would hold an edge. Also, by constantly "hitting" grass, small twigs, gravel, the edge becomes work hardened which also will
decrease the "toughness".
 
(quoted from post at 13:10:18 07/24/18) The load needs to open the crack tip (in this case the leading edge) which gets developed in three ways.

1. Tension loading radially due to the spinning blades (no moment, but still a fatigue load). The stress range in this case is 0 to full stress at a particular location at the full speed of the blades. The number of stops and
starts is your number of cycles. Note in this setup, tension is not constant; it is zero at the free edge (tips) and maximum near the bolt.

2. Varying load due to the blades hitting air (wind resistance); hitting grass; or in extreme cases hitting say a small tree. This will cause a bending moment on the blades, which also causes tension in the leading edge due
to this moment (note that when the mower is loaded the speed changes the tension as mentioned in part 1 so the stress range due to this mowing "moment" is amplified). In theory, ever blade of grass creates a small
stress range, but you also could look at the speed of the mower due to changes in resistance and create stress range and number of cycles that way as well.

3. Bending and tension caused by vibration due to the out of balanced blades. Here the number of cycles is directly related to the frequency of the vibration.


A few other points:
The way fatigue design works is the previously mentioned S-N curves (Stress verses cycle) curves, plots the maximum number of cycles at any given stress range. Using the previous equation for damage, you can see
how much life as a % is used at any stress range and then add the damage together to estimate the service life.

For most steels there is a minimum stress range to cause fatigue, however, with such large "cracks" in the serration and the expected hardening of the leading edge of the face, makes this a highly likely area for rapid
fatigue damage.

There is a case of "low, cycle fatigue" where at extremely high stress ranges, the life goes from thousands of cycles, to tens of cycles. This would be the case if the mower were to be spinning fully and come to a dead
stop by hitting a large unseen object. A few times will rapidly increase the crack growth.
Click to expand...

bjb it appears to me that all three of these loads would be greatest at the pivot point and least at the tip. It appears that the radial load would exist ONLY at the back of the pivot point. Is that true
 
In general the loads and stresses for the blade would be greatest near the pivot point.

The radial load exists as an almost linear decreasing function (assuming a uniform cross section of the blade) where the max load would be near the pivot point, approximately half that load level would be a the mid point,
and the load at the tip would be zero.

Note, all these discussions about loading is qualitative in nature, b/c the base assumption for discussion is a nice straight blade with uniform cross section (which makes it easier to picture the general fatigue and loading
aspects), the actual stress at any one location could increase or decrease based on any taper in the blade, bends in the blade, and the varying depth of the serrations.

Finally, notice I try and state the load is largest "near" the pivot pin. If this pin is tight (or binds), and there is friction between the blade and the housing disc, that will help transfer some of the load before the pivot pin,
which reduces the stress on the pin and in the blade at the pin location.
 
(quoted from post at 05:08:47 07/25/18) In general the loads and stresses for the blade would be greatest near the pivot point.

The radial load exists as an almost linear decreasing function (assuming a uniform cross section of the blade) where the max load would be near the pivot point, approximately half that load level would be a the mid point,
and the load at the tip would be zero.

Note, all these discussions about loading is qualitative in nature, b/c the base assumption for discussion is a nice straight blade with uniform cross section (which makes it easier to picture the general fatigue and loading
aspects), the actual stress at any one location could increase or decrease based on any taper in the blade, bends in the blade, and the varying depth of the serrations.

Finally, notice I try and state the load is largest "near" the pivot pin. If this pin is tight (or binds), and there is friction between the blade and the housing disc, that will help transfer some of the load before the pivot pin,
which reduces the stress on the pin and in the blade at the pin location.
Click to expand...

So...in other words, the chances of a 'catastrophic failure' from grinding a series of small fillet radius notches in the portion of the blade furthest from the pivot point is very slim....just what I figured. :eek:
 
I've seen some really butchered cutter blades and can't remember seeing any with the cutting tips broken off,theory is one thing practical application is another.
 
Worrying about the blade breaking because of cutting a few very shallow notches in it is waaaaaaaaaaaay down on the list of things to worry about.Great idea BTW think
I'll 'take the chance' on one of my cutters.
 
Well you did say this was an experiment, so record your hours of use with this blade set and when you replace them (if they haven't failed first), get them magnifulxed and see if any cracks formed in the leading edge.

If they do fail, take some good pictures of the cross section of the blade where it breaks and we should be able to see the growth of the stable crack and the point of brittle fracture (and the approximate critical crack
length).
 
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