Looking for solution to incorrigibly jumpy needle roller bearings

I have four Stihl hedge trimmer cutting heads of the kind you can see in a recent Youtube video put up by someone who undertook the repair of one.

The weak point in these gear boxes is the two con rods with their three dozen or so loose needle bearings in each: the individual needles jump out of position at the least provocation. You can see the result at 1.54 in the video.

I have looked into a total of eight of these gearboxes now (four belonging to other people) and all the con-rods I looked at had displaced needles; in half the cases some of the needles had leapt completely out of their races and were free to travel around in the gearbox and mash themselves into the gear teeth etc.

Sadly, what the chap in the video doesn't realise is that it is practically useless paying for new conrods (currently at £25 a pop) unless you're also going to buy a new spur gear (at £50) because if the needles were prone to jumping out of the race they sit in and causing havoc when the gearbox was new they're even more prone to doing so when there is extra lash between a worn spur gear and the new bearings.

My question is this:

would it be feasible to retrofit a better kind of bearing to these gear heads that will not be so prone to disintegrating?

Obviously the ID of any replacement bearing would have to match the OD of the raised areas of the spur gear that sit inside the con-rods, and it would have to be possible to achieve a good tight fit for any replacement bearing inside the (bored-out?) con-rod.

I would recommend putting no more money into repairing Stihl gas engines or associated parts. I suggest looking into imported cordless hedge trimmers at your local DIY big box store. Not sure about pricing in the UK at DIY stores but last year I bought an imported cordless hedge trimmer here, a local hardware chain's brand, Mastercraft, for about the equivalent of 55 UK pounds, with two batteries. Does a beautiful job, runs for over an hour on one battery, and if it only lasts a few years it is still cheaper than one repair's worth of parts on any Stihl machine (parts are staggeringly expensive for Stihl machines here). Stihl quality has dropped badly in the last few years, carburetors in particular. Just food for thought.

I'm inclined to agree with Jeff. I have a 400 series stihl chainsaw and a pole saw both of which have been fine but the smaller shihl chainsaw i used to own was a PITA to get started - not what one expects from a supposed quality brand.. I sold that and bought a cheapo petrol chainsaw from Lidl that has done all the smaller cutting since, starts very easily and cost less than a 1/4 the price and is in it's 5th year - gets used a lot.. The pole saw got damaged at one time when I couldn't get it out of the path of a falling branch in time and the repair cost for a bent pole was staggering. I have a lot of trees and woodland and a wood burner.

I used to have a petrol hedge trimmer.. not a stihl .. but heavy in use. I bought a cheap cordless one (another Lidl job) cynically expecting it to be useless but strangely even though it's reciprocating speed is way slower the quality of the cutting blades is brilliant, the whole thing weights very little, the 2AH battery lasts long enough for me to cut my 30 yard privet both sides and top twice without a rest period - so far second year of using it so only cut that hedge 7 or 8 times but if I had to throw it away next year and replace it'd still be better value than the petrol job it replaced with its frustrations, mess and costs.

It's a shame the cheaper cordless pole saws use skip chains - cut slow and vibrate and I don't use mine much unless it's a small job..

pgk

I have an older Stihl hedge cutter, electric motor driven. The con big ends have plain bearings (no needle rollers) It has been working hard for over 20 years and still carries on. the big ends and crankpins are slightly worn, it gets hot but still carries on, much like its operator.

Alan

Thanks for the replies.

Jeff and pgk, the hedge cutters are used commercially, often on very uncivilized hedges that have got way out of hand. I fear the £55 Mastercraft or the Lidl offering would not "cut it" on some of these, and certainly not be much use with only one battery with a one hour run time.

Stihl and other big names such as Echo and Husqvarna do offer more viable cordless alternatives for commercial users but they currently work out ludicrously expensive relative to their gas-powered counterparts; worse, they come with no guarantee how long the working life will be of the £300+-a-pop batteries. We'll all be forced to go electric eventually, but at the moment choosing cordless doesn't make sense economically for the commercial user. Which probably explains why I've not seen a single landscaping outfit in the flesh that has gone cordless.

Alan, yes, this terrible bearing design is not universal on hedge trimmers by Stihl, whether electric or gas-powered.

For the record, or on the remote chance that such a thing exists and someone here knows where to get them, the replacement bearings required would have an ID of 26mm, OD of 30-32mm, and a thickness of 6mm.

EtA: This appears to be the current style of bearing fitted.

Edited By Bill Phinn on 17/08/2021 19:43:58

Polymer plain bearing, see how long that lasts.

IGUS

Thanks, Roger.

There being one that doesn't require re-sizing would be preferable.

Out here in the sticks unruly hedges get a serious drubbing from a 90HP tractor with flails. I doubt your stihl would touch them.

I am seriously impressed with the cheapo cordless and double size batteries are available and not that expensive to own 3 or 4 if commercial or if away from power can always use an inverter on the car. Obviously don't know how bad your hedges really are but might be worth the punt with a trial.

pgk

the 26x32x6 dimensions indicate a F236696 bearing. which is still a full complement needle roller bearing. You could use these, or may be you could go to a thin section ball bearing like a 61705, that's only got a 4mm width and the load rating might be marginal.

Other than that, it's time to put the crank and/or the connecting rods in the lathe and make them fit something else.

Thanks, pgk and Mark.

pgk, we too have hedges out in our local sticks best tackled with a tractor-mounted flail, but in the immediate conurbation around me there aren't too many places a tractor and flail could sensibly be deployed; chainsaws, pole saws and hedge trimmers have to suffice.

Can I take it the cordless example you have experience of is a long reach hedge trimmer rather than one of standard length? The hedge trimmer that is the subject of this thread varies in length between 2.6 and 3.6m, and has a head that articulates through a 135 degree arc. Any worthwhile substitute for it would have to have a similar reach at least, regardless of other aspects of its performance.

Mark, the first bearing you linked to is the one I linked to and is fitted as standard in these gear heads, so definitely not a contender. I've also explored thin section ball bearings. I fear a one-third undersize width of 4mm would prove unsuitable.

Sadly not a long reach cordless.

I think that the Lidl stuff is usually made for them by einhell but checking their site the only electric long reach is corded.
When the power company sends guys round to do the 200yds of hedge under the powerlines to my house they've just used an ordinary pole chainsaw to get reach through a mix of hazel, damson, blackthorn and rowan. But then it's not topiary work. I have done the sides with loppers and hedge trimmer in the past (but before I owned the cordless)

I see your point for fancy neat high hedges around caravan parks and the like. Moving a tall orchard ladder every few yards is a PITA.

pgk

I recently had to sort out a non-running Stihl brushcutter at work. The problem appeared to be the carb, so a non-original "service kit" containg a complete new carb, spare priming bulb, fuel lines, fuel filter, air filters & spark plug was purchased from a UK Ebay seller for around £11 delivered. The carb was obviously not the same make as the Stihl branded, Chinese made, "Zama" original, but it fit & was straight forward to set up to get easy starting, a steady tickover & stumble-free transition to full throttle.

While this listing may not be the correct model of hedge trimmer & is for a direct-from-China seller, a kit of all the parts I think you are talking about isn't outrageously expensive. This link is for a pack of 4 conrods with bearings is £24 delivered.

Nigel B.

I would suggest that gearbox/motion translator failure is simply down to poor or non-existent lubrication. Nothing else. The needle roller bearings have failed before the hardened shaft on that gear, I suspect. First we need to know if the gear shafts have actually worn to outside the bearing clearance limits. Without measurements, satisfactory repair is a total lottery - unless all wearing parts are replaced (in which case the repair is likely uneconomic).

At first, I thought they had used non-standard bearings, but clearly(?) they have used a stock bearing. Buying the whole assembly is twice the cost of fitting new bearings. That may result in a repair cost being reduced by about 75% in cost of parts.

My old, approx 1kW, jack hammer runs by a bevel drive with throw of less than 2”(?) using a 7/16th x 5/8” x 1/2” needle roller bearing in the con rod. Clearly, from that video, many are simply incapable of following clear installation instructions for drawn cup needle roller bearings. That was yet another ‘amateurish’ video that made me cringe at his repair methods.

Watching that video was only of use for those that don’t have a diagram showing the construction of the item. For anything else, it was pretty useless and misleading to others, should they follow his utterances. Little wonder that subsequent failures often occur after a short operational period after ‘repair’.

not done it yet

Er what video are you talking about?

Please put in a link.

Thanks

Please put in a link.

The OP has a link to a video on the second line of the first post ?

Nigel B.

Thanks, Nigel.

Looking at the video I do wonder how those rollers managed to escape without major, visible, damage to bore or housing to make space for an escape route. Whatever it does suggest that a roller bearing is not the best type to use there.

I imagine the loading would be quite high when the blade hits a larger branch or gets jammed so a plastic bush, whether Igus or another breed, may well deform under it. I have seen such failures but the loads involved were certainly well beyond what the bearings were designed for.

I've had good results with the thin wall, wrapped, dry bearing material bushes. Glacier DU in my case but there are other suppliers. The surface speed is well within dry operation parameters and the thin bearing material on rigid steel backing plate is much stiffer than plastic. A filler piece round the outside will be needed. But that's a simple turning job. Perhaps a needle roller bearing inner race sleeve of suitable size could be found to press onto the gear giving a really good surface for the bearing to run on.

I'd probably shove an oilite bush in and see what happens.

Concerning battery hedge trimmers I've been very impressed with my 36 V Makita. Not quite as husky as my gardener mates petrol driven Stihl but, after trying it, he reckoned he could do quite well enough with one of those backed up by his small chainsaw to tackle the heftier bits. Said that in all honesty anything more than a little beyond what my Makita could handle was starting to abuse the Stihl. Pushing into "can do in an emergency" range with obvious consequences in shortening its life if done tto much.

Clive

Thanks for the further replies and info.

pgk, whilst long reach hedge trimmers are great for allowing you to stay on the ground and still cut tall hedges, this only really applies to A-shaped hedges; to cut the tops of flat-topped hedges you still have to get off the ground yourself if only in order to see what you’re cutting. The long reach cutters do, however, allow you to easily cut right across a wide hedge once you’re up there.

NDIY, you may be right about the lubrication. However, given that in all the examples of these gear boxes I’ve examined there was as much (or more) good quality grease in the gear boxes as when the machines left the factory I’d want to qualify your statement: if lubrication failing to protect critical surfaces was the cause of bearing failure in the examples I’ve handled, then that failure can only be laid at the door of the manufacturer for designing a gear box or recommending a grease that did not effectively distribute lubrication to the places that needed it.

I’ve voiced my own reservations about the video, but I’m not quite sure what you objected to. What did he get wrong in installing either the new conrods or the 6x10x9 bearings in the top and bottom of the gear case? I wouldn’t have used a hammer directly on the small bearing like he did, but he got them the right way up, didn’t he?

Nigel, yes, the carbs on Stihl stuff can be temperamental. One thing I’ve discovered only recently is the value of doing a leak-down test on two stroke carbs before you go changing the diaphragms, gaskets etc., or the whole thing. I found that all three of the carbs I’ve had on one piece of Stihl equipment were not airtight at either the metering end or the pump end or both. Curing that can help cut down on guesswork. I’ve not yet found non-OEM conrods or spur gears of the kind needed in these particular gearboxes, unfortunately.

Clive, I’m not quite sure how to understand your third paragraph. When you say a filler piece is needed around the outside of the bush, is this simply because no bush of the right dimensions is available in this case? I’ve not yet fully explored the sizes available so I’m not sure what choices I have. Mark’s earlier suggestion of a thin-walled bearing is also still attractive as long as I could get one of a width nearer to the required 6mm than 4mm, or somehow compensate for this dimensional discrepancy.

Interesting to see that Stihl use a caged rather than an uncaged bearing in the conrods of one of their more recent hedge trimmers. I suspect I don't need to be told why.

If you can find ball races that will fit the shaft and the housing, you could always replace a long , unreliable needle bearing by two, or more, ball races with suitable spacers. To take the loads, it would be worth fitting as many ball races as possible, to carry the loads.

The end product might be more durable than the original equipment!

HTH

Howard

Bill

For the same internal diameter as the original bearings the thin wall wrapped dry bearing material bushes are a smaller outside diameter. So you will need a filler piece to make up the difference.

I've cut down thin wall bushes in the past with no great issues.

Clive