Bosch GRA53 36V Lawnmower - no motor

[stj]

Re: Bosch GRA53 36V Lawnmower - no motor

can you meter inductance?
taking a Q reading from each winding would be good.

[DanielCoffey]

Re: Bosch GRA53 36V Lawnmower - no motor

Here is an update for the curious...

I managed to get the lower circlip out that held the lower bearing in place (in 2021 I fitted an SKF 25x47x12 Metal Shielded High Speed). There was a load of rust and the bearing had actually rusted solid. I deformed and levered the metal shields off and the contents were jammed with dirt and rust. After a soak in WD40 and a scrub with a brush I managed to rock the bearing free and flush it again.

Of course the mower did not function without the motor fitted. The LEDs on the two control boards gave three fast flashes and shut down. Adding the cap with the Hall sensors did nothing new. Adding the motor body without the rotor did nothing. There is definitely a feedback from the Hall sensors that is required for power to be supplied to the motor.

Since all I wanted to verify was the control boards I reassembled the motor with the rusted bearing dry. Even a twitch or attempt to turn would show it was being controlled. The upper bearing was free and dry. The thrust spring and end cap went back on fine. The motor was free to turn although it was very rough.

Connecting the motor and sensor leads back to the mower gave exactly the same result as when the mower was siezed. Not a twitch out of the motor even though it is now free to turn followed by a whine, clicks and shutdown.

I connect the battery, press the master button and the red lights on the control boards turn on. Depressing the drive bar while lifting the rear allows the rear drive to engage and be controlled so that control board still works. The fault of a high-pitched whine for a few seconds and three clicks before shutdown is still present.

I removed the motor and checked continuity between the three legs. All three are low resistance (under 1 ohm). None of the legs make any connection to the case of the motor so there is no low voltage short.

I won't be able to find any information as to what the test or programming pins on the controller are for or what the clicks mean.

So, other than "the motor control board is knackered" are there any other ideas before scrapping it all?

[CapLeaker]

Re: Bosch GRA53 36V Lawnmower - no motor

oh well, since it appears to be more or less a total loss I'd still just grind it down so that it will at least turn and detect, even if it's not perfect, alas it's up to you. Not sure how long my motor will last, at least I try not to mow when wet.

Hahaha! I mowed 2 times this year dry. All the other times it wa soaking wet to the tune the rear finishing mower deck sunk in on a 4wd tractor. The zero turn would have totally got stuck down there. Our weather is the shits. Never seen anything like it here.

[eccerr0r]

Re: Bosch GRA53 36V Lawnmower - no motor

oh well, since it appears to be more or less a total loss I'd still just grind it down so that it will at least turn and detect, even if it's not perfect, alas it's up to you. Not sure how long my motor will last, at least I try not to mow when wet.

[DanielCoffey]

Re: Bosch GRA53 36V Lawnmower - no motor

I'd imagine it should still work somewhat with the crack there? might run funny but should rotate.

Unfortunately the cracked stator was preventing the blade from turning. Considerable force was needed to even get it to move. There was also signs of carbon on the inside of the motor and end cap. The Hall sensor board for monitoring rotation speed was covered in carbon too.

There is a slim chance that the Honda HRX 476 XB will use the same motor as it is based on the Bosch GRA46 (the smaller brother of what I have been using). Both mowers use the same battery format as Honda bought the rights from Bosch. There are no parts diagrams of the Honda yet as it is too new but I can see the motor has the same housing as the Bosch one.

One major difference that is likely to be a showstopper is that the Bosch has a short shaft with the blade bolted directly to the end and moves via an independant rear axle electric motor. The Honda has a longer shaft with a belt drive pulley to the rear wheels. They are going to be incompatible I suspect.

I am using the Honda HRX 476 XB at the moment but there are several important differences. The cut is 46cm vs 53cm and I do have a large garden. The Honda uses the same batteries but takes only a single one at a time. While it is a matter of seconds to swap the battery (I have 4x 9Ah) the fact it draws from only one battery means it gets hot quickly. The Bosch used a pair in parallel so they ran cooler which was better for the batteries. Finally the narrow wheelbase of the Honda makes it jerkier over rough ground which is more effort to steer.

The reason I picked the Honda over other electric mowers is that I am invested in the batteries and two of them were very new with only a few charge cycles on them. The Honda does come with a 7 year non-commercial warranty which means I can use it until these batteries are worn out then consider changing to something completely different at that time.

[eccerr0r]

Re: Bosch GRA53 36V Lawnmower - no motor

I'd imagine it should still work somewhat with the crack there? might run funny but should rotate.

I still use a lead acid mower that has a permanent magnet brushed motor. It's cheaper than the gasoline mower because I got it for the repair cost (was given to me nonfunctional with working battery). Was able to repair it and it's way cheaper than gas, minus the fact it takes longer to mow since it's a smaller mower...

I imagine the lithium ion mowers have enough specific power/capacity to run a wider cutting blade, but it's great not to have to change oil, fight with a clogged carburetor, deal with a cord, or buy gasoline...

[petehall347]

Re: Bosch GRA53 36V Lawnmower - no motor

most of the broken ones i look at are not viable to repair . the control boards cost a lot or are not available .
the 80v stuff i dont have to look at too much . with them its batteries and chargers .

[stj]

Re: Bosch GRA53 36V Lawnmower - no motor

but they cost 3-5x as much

[DanielCoffey]

Re: Bosch GRA53 36V Lawnmower - no motor

Thank you but I disagree. Battery powered motors can handle fairly large areas now and are quieter and cheaper to run than petrol. Some of the higher powered ones can tackle anything that the larger single cylinder petrols were used for. There are some good all-electric riding mowers too that are based on Lithium.

[stj]

Re: Bosch GRA53 36V Lawnmower - no motor

should have got a mains powered mower.
or petrol if your doing a large area.

[CapLeaker]

Re: Bosch GRA53 36V Lawnmower - no motor

Wow! Well that sux having a cracked motor!

[DanielCoffey]

Re: Bosch GRA53 36V Lawnmower - no motor

Well to be fair that rust was inside of the motor. The rest of the machine is fine. The body is powdercoated alloy and the fittings are stainless or coated.

I think most of the water leaked in past the cheap shaft seal and once inside started rusting the body of the motor. I suspect the cores in this type of brushless motors are simple iron or steel anyway.

While I could get the motor rebuilt and a new shaft seal fitted I don't want to spend that kind of money to find the electronics might not work. This is the unfortunate consequence of being an early adopter for a brand. This range of tools was a huge failure for Bosch and they dumped it all on Honda. Nowhere has original spares now as it has been discontinued for almost four years.

[stj]

Re: Bosch GRA53 36V Lawnmower - no motor

it's a pile of crap if it's not stainless or galvanised or coated in some way.
who the fuck sells a garden tool that can rust?!

[petehall347]

Re: Bosch GRA53 36V Lawnmower - no motor

that will be your problem but without a new one or known good one you dont know if the electronics still work .

[DanielCoffey]

Re: Bosch GRA53 36V Lawnmower - no motor

Well folks - it is moot... the motor itself has failed due to corrosion and has a crack right across the stator.

It looks like there has been considerable water ingress over quite some time through the "shaft seal" (which is really only for impact protection). The lower bearing is rusted almost solid and is reluctant to turn. The top bearing under the end cap is in good condition but wet.

The permanent magnet rotor is wet, surface rusted but intact. A scotchbrite pad would have cleaned it up fine.

The stator with its 12 windings still has good wiring but the laminated core has a crack running completely through it from top to bottom. It is heavily rusted and shows signs of burning with carbon on the end cap. The failure of the stator is what was binding up the entire assembly.

Now it is possible I might be able to find a replacement stator/rotor core for the motor since I suspect it is a nominal size of 100mm but it looks like it would need a press to install it in the proprietory Bosch motor housing and I certainly don't have one.

Here are two pics - one of the complete housing and one of the damaged stator.

[sam_sam_sam]

Re: Bosch GRA53 36V Lawnmower - no motor

SKS is a series that has only a few sizes if I remember correctly but if you are going to be using them out get the sealed bearing series for best results

If they have a high speed version this might be a better option and if it is a sealed bearing

[petehall347]

Re: Bosch GRA53 36V Lawnmower - no motor

take it they are open bearings ? try 2rs ones . not heard of sks . skf is a good bearing ..

[stj]

Re: Bosch GRA53 36V Lawnmower - no motor

get stainless or ceramic bearings next time.
never use regular steel for anything going outdoors - not even screws.

[DanielCoffey]

Re: Bosch GRA53 36V Lawnmower - no motor

This is the Shaft Seal that was covering the most exposed bearing. I was wondering if it was incomplete but I suspect it is just cheap.

When the mower was originally about two years old I noticed steam coming out from under the mower when cutting long and damp grass. The blade was turning roughly and this was the exact shaft seal that was in place at the time. There did not appear to be any rubber around the seal and there was about a 1mm gap between the seal and the shaft itself. Dirt and water had got into the original bearings which was causing the heat that was making the wet grass steam.

I had to use a screwdriver blade between the motor housing and the sides of the shaft seal to collapse them in so it would release. At the time I replaced the bearings with SKS brand, straightened the sides of the seal and repositioned it.

The same failure has happened again so I am going to be measuring the OD, the shaft diameter and seal thickness and upgrading it.

Any suggestions for what type and material of seal to get assuming I match the original dimensions?

This is a seal for a vehicle which while the incorrect size shows the sort of thing I think I should be measuring and using. I suspect the part will be 35mm OD, 25mm ID but will measure with digital calipers once everything is clean.

https://www.japanese4x4parts.co.uk/t...mm-19650-p.asp

[petehall347]

Re: Bosch GRA53 36V Lawnmower - no motor

yep get the motor turning first .