The front shaft support bearing on my front deck lawnmower keeps burning out its seal.

Typical scenario: PTO coupling catches long strands of grass which wraps around the PTO coupling, and also around the splined shaft between the coupling and the bearing. Friction heats up the plastic seal, it falls out, then the back up metal retainer falls out. Soon after the bearing is history.

The bearings are cheap, but it takes about 2 hours of awkward work to replace them. (I'm considering using a split pillow block to make the replacement easier.)

Only recently figured out the connection between long grass and bearing failure. On this particular occasion, the failure was about 2 hours after the long grass.

Q1: What do I ask for to get a bearing with a seal that withstands more abuse?

Q2: Are there other ways to protect the bearing seal?

Edit: As per request: Grasshopper 1995 721D

Rear bearing never winds grass.

Mower is NOT used for a lawn, but to maintain aisles in a tree farm. Some of this is done with a John Deere MX-6 rotary disk mower (6 foot wide; run by a 55 HP Deutz Tractor. The grasshopper is in effect being used as a 'trim' mower, and is used for certain classes of weed control once a year, as well as mowing trails in the woods. (Snowberry, currants, raspberries, hazelnuts, baneberry, elderberry It also mows lots of gopher hills. Part of it's task too is to mow the oil lease access road twice a year -- another mess of clover, alfalfa, thistles, and grasses of types that embarrass creeping red fescue as being a weakling come lately.

I am not trying for a manicured lawn. I'm trying for a 'trimmed pasture'

  • $\begingroup$ Cut the grass before it gets so long... Make sure the blades are sharp... $\endgroup$
    – Solar Mike
    Commented Aug 28, 2019 at 4:13
  • $\begingroup$ Sharp blades are good for about 3 gopher mounds. Cutting weeds everytime they reach 8 inches is not in the cards. $\endgroup$ Commented Sep 5, 2019 at 13:02

4 Answers 4


Clamp a split lock collar around the PTO shaft right next to the face of the bearing so grass cannot rub against the bearing seal.

  • $\begingroup$ I like this idea. I was wonder too: I have some 0.25" HDPE. Cut a washer out of that, and use the split collar to hold it snug against the face of the pillowblock. $\endgroup$ Commented Aug 28, 2019 at 22:08
  • $\begingroup$ yes yes! sounds worth a try. $\endgroup$ Commented Aug 28, 2019 at 23:59
  • $\begingroup$ The heat isn't coming from the grass. It's migrating down the shaft from somewhere. Check for exhaust leaks, missing heat shields, worn universal joints, failed motor mounts and the like. $\endgroup$
    – Phil Sweet
    Commented Aug 29, 2019 at 0:41
  • $\begingroup$ @PhilSweet, excellent suggestion. If this is a shaft that emerges from a gearbox, it could be bad lube or wrong lube in the box. $\endgroup$ Commented Aug 29, 2019 at 3:15
  1. Meticulously clean the mower after each use. This means hosing down the underside until no grass is left.

  2. Let the thing cut the grass. If you are birdsnesting grass you aren't letting it cut. Slow down, and yes, sharpen the blades - every four hours if in deep grass.

  3. Assuming this is a shaft running to a gearbox on the deck, make sure you don't have any loose hardware on the deck system, the lift system (check for worn bushings), loose/worn/missing outboard carrier bearing on the universal stub shaft. Grease the universal joints. These are normally a royal PITA to get at. Grease all chassis points every 50 hours and all blade spindles/deck casters/rollers every 10 hours.

(I run mowers and fix them at the local golf course)

  • $\begingroup$ Oh, It would help is you listed the year/make/model of the mower and posted a picture of linked to one. $\endgroup$
    – Phil Sweet
    Commented Aug 28, 2019 at 9:15
  • $\begingroup$ 3 deck spindle zirks, 2 PTO zirks, 6 wheel zirks. Doing the underside of the deck requires a winch, or the use of a front end loader bucket. Blades get resharpened about twice summer. Normal use pattern is aobut 12 hours per week. Deck has never been cleaned. I think the dirt holds it together. $\endgroup$ Commented Aug 28, 2019 at 22:12

This is an incomplete answer, but may help other people.

Learned lots yesterday.

Split pillow blocks aren't made in small sizes. Surprised me.

The bearing supply house I went to (Motion canada) didn't have bearings with metal seals in the size I needed.

They also didn't have a narrow enough collar to be effective on the central bearing shaft. It protrudes only about 1/4" beyond the pillow block.

Nisku Nut & Bolt did have washers that were about 1/16" larger than the center shaft of the bearing, and a few thou too big to nest nicely in the ledge on the pillow block. Some work with a sander fixed the latter.

I placed the washer over the protruding bearing shaft, made sure it had clearance all the way around (10 to 20 thou?) and stuck it in place with bathtub silicon seal in the crack between the washer and the pillow block.

Potential failure modes:

The space between the face of the washer and bearing is on the order of 1 mm (40 thou) The washer is stationary, the bearing rotating. The outer edge is more or less sealed, the inner edge of the washer open. This may act as a dust collection centrifuge. I'm debating whether to over grease this enough enough fill the space with grease.

If the bathtub silicon lets go, I will have a washer ratling lose on the shaft between the bearing and the PTO. I don't think this is a big worry. There is nothing in that space to bind.

  • $\begingroup$ Did you look on McMaster Carr for the split collar? They have a ridiculous selection. $\endgroup$
    – Drew
    Commented Aug 30, 2019 at 19:14

I think the conclusion is probably wrong. You are assuming the seal falls out then the bearing gets wrecked. In my experience it is the other way round the wrecked bearing generates heat and the plastic seal falls out.

The long grass should not be generating enough heat to damage the seal but it is probably putting load onto the bearing and wrecking it.

  • $\begingroup$ I don't think that heat is the issue as much as abrasion. In the last instance, both the plastic outer seal and a thin metal inner seal were both gone. I could see the race containing the rollers. So far my made shift solution has run for about 10 hours using the damaged bearing. I will check the bearing heat next time I use it. $\endgroup$ Commented Sep 5, 2019 at 13:01
  • $\begingroup$ At the time I removed the last batch of long grass, the friction was still low enough that rotating the PTO by hand (which meant the entire PTO back to the clutch, and forward to to the blades) was still easy to do. $\endgroup$ Commented Sep 5, 2019 at 13:05
  • $\begingroup$ A collapsing roller bearing is not necessarily hard to turn. It's not like a bush. or shell bearing. However you are the man on site I can only hazard guesses at what is happening. $\endgroup$
    – Brad
    Commented Sep 5, 2019 at 13:14
  • $\begingroup$ My point was that the grass wasn't creating much friction. The deck uses at a guess at least 1/3 of the HP of the engine, so 21/3 = 7 HP = ~4KW At, say 600 rpm, you have 10 rps, which is about 400J/revolution. An extra load to cause bearing failure should be on the order of that same load, yes? $\endgroup$ Commented Sep 5, 2019 at 14:35
  • $\begingroup$ It is hard for me to say without seeing the item you are talking about and the actual use, but bearing failure often happens because of a few simple reasons which are not directly load related. Wrong lubrication, side load, end load, on the bearing itself as opposed to load = torque on the machine, you did say you were using it on a rough vegetation. $\endgroup$
    – Brad
    Commented Sep 5, 2019 at 15:01

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