OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Four Stroke Lawn Mower Frames Masport Blade Disc - Damage/ Problems/ Failure

More recently I have encountered a number of newer Masport mowers with 4 blade disks that have shattered from some sort of stress fracture in the centre of the disk and completely fallen off the crankshaft obviously while the motor is running however I cant be 100% sure because this is how I find them at the recycle yards. I am curious if I have just been unlucky as I have found 3 or 4 over the last month. This problem only appears to occur with the four blade disks and not the 2 blade disks.

Up until this point I have replaced the damaged disks with the 2 blade set up and haven�t had any problems. The cranks and bosses don�t have any notable damage with the exception of a couple of snapped crank shafts which I will mention later.

I am trying to understand why this is becoming a common occurrence (if others think so) as this disk seem to shatter and break off as wide as the washer used underneath the bolt that secures the blade disk to the crank shaft.

Thoughts so far.

- Blade disks not being torqued correctly to the crankshaft
- Blade plate is too heavy.
- Blade disks hitting a hard object i.e street gutters/ rocls. I would think this would be more common with bar blades and not disks.
- I thought the cranks might have been bent however with a replacement boss and plate I can confirm the cranks were not bent. (My good spares are running low)

I have found reference to a Masport blade plate stiffener. Is this the key to ensuring reliability and ongoing safety?

On a more extreme note I have found 2 relatively new masport mowers (2-3 years old) with Briggs Engines where the crank shafts have snapped off completely. These are great parts mowers but that�s about it.

Any thoughts or experiences would be appreciated.

Thank you

Jaffa

I haven't heard of the four disk plate problem but have come across mowers with snapped cranks with the single blade type cutter bar. One was a brand new mower that hit a small garden stake on its very first outing. Of course there is no warranty cover but it is terrible design that they don't at least have some sort of friction washer that will take most of the impact.

I guess this is where the Chinese have cheaped out. When you say snapped cranks is that internal or external?

There was a recall on at least some of these Masports. Details: http://masport.com.au/news/article/product-safety-recall-au

Norm, The cranks usually snap just above the boss. Trev might have found something different. I have only ever found snapped cranks on the Masports and not Rover or Victa�s.

I have a 625 Briggs at home with a crack in the shaft just above the boss. I�ll try to remember to take a photo of it tonight. Again this one came off a Masport.

Gadge, Thanks for the confirmation on the recall. The couple I have are 2010 models. So essentially the damage is because of people hitting let�s call them un-identified objects.

Does anybody have any experiences with the blade stiffeners? If I come across a Masport that falls under the recall I�ll be making sure they have the stiffeners fitted.

Jaffa, is it a Masport problem or a Briggs problem?

Norm, the units in question have all been Masport's and all relatively new models. To date I haven't seen any snapped cranks on Victa's or Rovers, that's not to say it hasn't happened.

I am still interested in peoples thoughts on the blade plate issues.

G'day all

I feel Gadge is on the money.
Masports are, of course, not made in NZ anymore.

Blade disc fracture seems to be the effect - and probably
affected by marginal design or marginal materials ... or poor
quality control. I don't know.

It is a dangerous situation when this component fails.

I have a personal story on disc failure...
As a teenager apprentice I was out with the Boss demonstrating an
X brand AUS ride-on. An incorrectly tolerenced cutter spindle caused
the bearings to seize in a dramatic way. The design enabled the inertia
in the blade disc to unscrew itself from said spindle.

The disc departed the cutter deck and became air born.
It missed four people by about a metre!

The issue was caused by poor quality control and poor design.
Very frightening...

I hope Masport have sorted the problem you describe.
Blade holders should be able to withstand any imbalance
caused by poor maintenance.

This is just my view.
-----------------------------
Jack

G'day folks,

I'd say that the blade disc failure is a Masport issue for sure. Design for a start, in that the original design didn't handle the shear stress concentration, which naturally occurs in the blade disc, around the periphery of the boss.

The crankshaft failures are a bit more complex, and could be down to Masport, B&S, or just an unfortunate combination of both.

Time to go into some OPE industry history, I feel!

Older US-made Briggs crankshaft PTO's would bend a fair bit when an object strike occurred, without fracturing.
Masport recognised this issue, when they made a big push into the Oz market in about 1975. That was just after they had taken over Whirlwind P/L.

Bar blades were not used by any of the then Oz push mower manufacturers, so bent PTO's were not a common issue, until Masport and their bar blades. Masport consequently required their dealers to buy a 'crankshaft straightener' [a bench mounted 'special tool'], which I have described elsewhere on ODK.

It was a 'crank out of engine' setup, and the usual bend point was just below the oil seal. The amount of bend usually wasn't sufficient to prevent removal of the sump, and these would straighten to within spec easily.

What I'm getting at here, is that older Briggs crankshafts seem to have been heat treated more for 'toughness' than 'hardness', on the PTO ends at least. These two properties are essentially inversely related; i.e. as one increases, the other decreases.

These fractures indicate that heat treatment for a 'tough PTO end' may no longer be the case, with the Chinese-made Briggs cranks. Decreased toughness/increased hardness results in a greater tendency to brittle fracturing, which is what we may well be seeing here.

The inner top edge of the boss forms a bending stress concentration point, by its nature, so that's why some of the failures occur at that point on the shaft.

I thought I should add some photos of the problems encountered to add some depth to the topic. This example cracked quite low but others have cracked well above the blade boss.

Any thoughts would be appreciated.

I'm only an amateur here, but even I'd say it looks like very poor quality metal that has resulted in catastrophic fracturing. This would be VERY dangerous to use. I have seen this happen before with bosses and even cranks, but usually after hitting something very hard. I cannot remember the brands.

The only bizarre issue I have had with blade discs was on a brand new Supaswift mower. For some inexplicable reason, one day when I was mowing the entire disc suddenly dropped off. It kept rotating and dug a neat hole into the lawn, making a bare round patch which took months to grow over again. I cannot explain it, because I always do the bolts up tightly to the required torque. The grass was perhaps a bit higher than usual and was a bit damp, but not overly wet. Anyway it was the first time something like that ever happened and it left me scratching my head. It has never happened again.

My opinion on Briggs motors has never been high, they might sell a lot of motors but that doesn't make them good

Yuck...I haven't seen anything like that here but have seen plenty of bent cranks from impacts.

I still have only seen this issue happen on 4 blade Masport mowers. This one snapped/cracked low and the other which I didn't get a photo of snapped higher.

My thinking is it might be related to the design off the boss and blade plate. I have never seen this happen on a Victa or a Rover.

Gadge will no doubt chime in.

As always I am interested in other thoughts.

The fact the shaft in this instance cracked around the key makes me think the fault is in relation to the design of the boss. Gadge will no doubt chime in with his comments on the photo.

Good pics there, Jaffa J.

I don't really think that's due to a design fault in the boss. The only way the boss could contribute to that sort of cracking, is if the keyway in the boss is oversize in width, thus allowing/forcing the Woodruff key to tilt in the recess in the crank, and thus overstress the top edge of the recess, when an object strike occurs.

Those corners of the recess are obviously the points where the cracks started, and propagated from.

Gadge you are correct but the reason it fractures is as you pointed out earlier it is a hardening issue to begin with. This cracking is a direct result of that.

NormK, agreed. Typical brittle fracturing! Too much hard, not enough tough.

As an aside, back in the 70's if a Masport PTO was bent too far to get the sump off, we wouldn't repair it.

It was cheaper then [and better engineering practice too] to just fit a new short motor, rather than smash the old sump off the crank and replace it.

Bending is what it should be capable of doing, a car axle will twist the spline a long way before it snaps

Gadge, Thanks for the clarification. It makes perfect sense.

I should note the other Masport's I have encountered (3 in total) have snapped well above the boss. Unfortunately I don't have any photo's of these.

This one was a little unique.

I will remember to take a photo of a completely snapped crank when I find another one. I just find it interesting that I have only seen this type of issue with Masport's.