In this Instructable I'm going to show how I repaired an old 'Weed Eater' string trimmer. New parts seem to be still available but I really didn't want to spend $50.00+ for a machine that only cost $80.00 (around 10 yrs ago) It had a pretty serious vibration problem for a few years,I assumed the engine was worn out but when drive stopped working motor still ran fine
With nothing to loose I decided to take it apart and find out how much damage had been done
I'm making a few assumptions that anyone reading Instructables will already have majority of tools needed, Dremel is now a generic name for mini grinders most of the other tools required are relatively cheap (except for Dremel, you could probably get everything needed for less tan $30.00) I've had my Dremel for at least 10 yrs, it's old and tatty but works fine (just like me :o) )
Bench vice (makes life easier but any sort of holding device can work)
Metric Allen keys
Caliper ((around $15.00, Harbor Freight, et.al.)
Thin steel (I found a bottle cap) and had one shim washer
Power drill or drill press
'Step' drill bit
'Flat' Screwdriver or small pry-bar
Dremel (or similar)
Diamond burr's 1/8" shank for Dremel
You only need the pointy 'tree' burr but full set from China is cheaper than getting single burr locally
Step 1: The Offending Part
Gear head on old Weedeater is 'engineering plastic' held together with multiple Torx head screws plus 3 Allen head screws/bolts clamping head to shaft (two clamp, one to prevent head rotating on shaft)..There is also a wing nut holding guard in place which needs to be removed to access the 'hidden' screw holding two halves of gear case together. Unfortunately I got carried away (never intended to make an Instructable out of repair) so there are no pictures of case disassembly.
The large 'nut' screwed onto shaft is also a labyrinth 'seal' being a close fit on the extended spigot of gear case.This was the only difficult thing to remove. I gripped 'nut' it in bench vice, removed screw holding string holder head retaining spring then unscrewed the 'cutter' head
Except for previously mentioned screw hidden under 'string' guard all the others are fully visible (after cleaning years of grass dust from the recesses ;o) )
Step 2: The Damaged Bevel Gears
Took out all the screws (after finding the one hidden under blade cover) Forgot to take pictures - sorry
The two case halves were very tight,'snapped' together maybe? I couldn't find any assembly clips though? Then tapped things with hammer from various directions but ended up using a flat screwdriver as a pry-bar to separate halves (I thought I had pictures but something must have gone wrong as I can't find them?)
I started re-shaping the driven gear (in vice) then decided to take some more pictures.
First picture, top 2:00 position has already been roughed out with burrs removed
Two pictures of drive gear from slightly different angle trying to show the burrs on teeth.You can see the depth of tooth remaining, adding shims between bearing and gear will 'lift' it into deeper engagement. It's an odd size though so I had to make a shim. I didn't take any measurements just 'eye-balled' the gears assembled in half of gear head and used what I found lying about. The bearing below gear had to be removed to add a shim underneath
Step 3: Making a Shim
Used a bottle cap as it was steel instead of aluminium soft drink can (I think it's off a 'Bud'?) I know aluminium will 'squash down' over time so didn't use it
Tried holding cap in drill vice, too flimsy and didn't work. Used bench vice to press cap 'teeth' into a piece of scrap pine.
When cap flattened off it was overhanging sides of wood so I had to find some bars to grip wood without touching cap edge.
Measured step drill to find something close to 0.470" diameter. Very surprised when the '4th step' was correct size
Picture of gear shaft in the hole, checked before removing from drill press - 'just in case'
Used a tin-snip's to cut shim roughly to size. After de-burring shim it fitted shaft pretty good. I re-fitted bearing after cleaning all the old grease then re-greasing with synthetic grease (I use it on motorcycle wheel bearings and any grease points - usually swing arm bearings)
Update. 31st Aug 2017
I did a bit of research on bevel gear set up (setting up cfrown wheel and pinion on rear wheel drive vehicles)
The 0.011" shim is too thin, I know I really should have checked gear mesh. I'm making a new shim 0.025" thick to increase depth of tooth engagement. Managed to get front yard 'mowed' though so all good
Step 4: Dremel and Diamong Cutter (burr)
I didn't know how hard the gear teeth were (probably tough rather than hardened, they would snap rather than 'bend' if real hard?) I had a set of burrs from a different project, most of the parallel ones were too big to fit between gear teeth but the 'tree' was unused and fitted a bit tight due to the gear teeth being rounded over. I roughly removed all the bent bits then checked teeth using a x10 loupe (I'm 60, can't see as well as I used to 40+ yrs ago- lol)
Used x2 loupe and cleaned all the gear teeth very close to original shape.(got pretty close to a tool spinning at 15~2-,000rpm) After both gears were cleaned up I tried 'running' one against the other. Seemed OK but obviously gears needed meshing tighter together.Using 0.020" and 0.011" shims under the gears seemed to give a 'snug' fit when assembled in gear case half.Packed half of gear case with extra grease plus added some motorcycle chain lube as it is a 'thin' grease capable of withstanding very high localised loads. Assembling the case halves was a slight problem, it was very tight coming apart. I used the screws to carefully pull the halves into position. The 'hidden' screw is visible just above my finger in the last picture
Step 5: Proof of the Pudding........... (video to Come Shortly)
Here's the vid.
Re-fitted gear head to shaft, the drive rod was easier to insert into drive gear than I expected.
I should have set the 'string' a bit longer but wanted to do a test run.
Video of it running coming soon, have to upload to You Tube before it can be 'embedded' here
It worked better than expected although the 'lock-nut'/seal moved 0.020" closer to plastic spigot so was rubbing a bit. .I sprayed some more chain lube in gap which seems to have cured the problem. After running I rechecked all the screws, as expected everything had aligned properly so screws needed ~1/3 turn to re-tighten. Took me just about 4~5 hrs to fix it, spent $9.99 on a replacement part that didn't fit but still saved $40.00+ and have gears that could last another 10 yrs (motor will probably wear out before that?) I also have a pair of reduction bevel gears 'spare' for some future project (always keep 'useful material' LOL)
This is entered in the 'Fix It' contest,if you think it could be useful repair please feel free to vote for it.