Black and Decker Circular Saw - UK Vintage Radio Repair and Restoration Discussion Forum

Jimbo · 24th Aug 2022, 10:57 pm

My Black and Decker PL40 circular saw has a fault that is more mechanical than electrical, but there are many contributors to this group with better engineering workshop experience than I, so I would be grateful for any guidance.

The motor runs powerfully and the blade spins, -until it contacts the workpiece, at which point the blade stops and the motor keeps running on merrily.

In the PL40 design, a helical gear at the end of the motor armature engages with the perimeter of a larger gear wheel with teeth that are roughly trapezoidal in profile. This larger gear wheel is mounted on a short stub shaft that runs in a bearing supported by the gearbox housing, and the stub projects on the outside of the housing to carry the blade itself. The large gear wheel is an interference fit on the stub shaft and was probably heat-shrunk on to it during the manufacturing stage. It is this ‘interference’ connection that has failed, and any slight resistance to the spinning of the blade causes the connection to break down.

The Black and Decker replacement part, [which comprises the gear wheel, bearing, stub shaft and housing as one unit] has been ‘unobtanium’ for a long time and I have missed out on a number of attempts to buy a ‘spares or repair’ saw with a duff motor but good mechanicals on Internet Auction sites.

I have dismantled the whole thing and there are some signs of scoring on the surface of the stub shaft so it is unlikely that the full transfer of torque will ever be possible without giving the joint some additional mechanical help.

There are proprietary ‘bearing assembly compounds’ available that purport to improve the fit of bearings on shafts but I don’t know that they are intended/capable of carrying the full shear stresses across the shaft to gear wheel junction.

My only ideas have been to try and file/grind a small slot in the stub shaft and the bore of the gear wheel and to then insert a home-made ‘Woodruff’ key, or alternatively to try and drill a small diameter hole through the gearwheel into the stub shaft and then to lock them together with a steel pin. I have a reasonable drill press and a vice that would hold things steady but I doubt that my normal high speed steel twist drills will make any headway in the gear wheel or stub shaft that are both likely to be of hardened steel.

I would be grateful for any advice

Jim M0GJD

Radio Wrangler · 24th Aug 2022, 11:35 pm

Without seeing the thing, commenting is difficult. Bearing assembly compounds usually break up if subjected to intermittent forces over a long period. Perhaps a couple of tack welds could be more robust? MIG or TIG would be fast and not too heating.

David

G6ONEDave · 25th Aug 2022, 9:54 am

Can't remember the type number but there is a Loctite anaerobic adhesive that will hold under rotational torque, it's a filler type. I've used it on gears that were an interference fit on the end of motor shafts previously.

Dave

thermion · 25th Aug 2022, 10:13 am

Loctite 648 bearing retainer is good for things such as this.
Another way is to knurl the shaft & press the gear back on.
Assuming the shaft isn’t hardened that is

Refugee · 25th Aug 2022, 2:41 pm

Studlock is the best.
Locktite used to call it 270 but now most of the types with 270 in the number are alternative brands.
The bottle I have got at the moment is called Bondloc B270.
Try someone like Screwfix.
Obviously clean the parts thoroughly with oil free solvent before you even think about using it.

Sinewave · 25th Aug 2022, 2:45 pm

Loctite retaining compound or bearing retainer, it's cutting edge stuff for things like this.

Jimbo · 30th Aug 2022, 9:11 pm

Thank you for your helpful comments.

I opted for the Loctite 648 solution as the shape and hardness of the shaft and gear wheel precluded any 'mechanical' fix that I could attempt in a home workshop.

I am impressed with the Loctite product! It's funny stuff.... a small globule on a plastic dish on the workbench was still a gooey liquid after 24 hours, but as soon as it is confined between two surfaces it cures and hardens quite rapidly. A few drops on an M6 thread, [which is a much 'looser' contact fit than a bearing on a shaft], bonded so well that it took a large pair of grips and all my strength to shift it after 24 hours.

The saw is now returned to working order and performed a 25mm deep cut in timber with no problem.

Thank you again.

Moderators, this thread can now be closed, thank you.

Jimbo M0GJD

24th Aug 2022, 10:57 pm	#1
Jimbo Pentode Join Date: Sep 2006 Location: Crewkerne, Somerset, UK. Posts: 114	Black and Decker Circular Saw My Black and Decker PL40 circular saw has a fault that is more mechanical than electrical, but there are many contributors to this group with better engineering workshop experience than I, so I would be grateful for any guidance. The motor runs powerfully and the blade spins, -until it contacts the workpiece, at which point the blade stops and the motor keeps running on merrily. In the PL40 design, a helical gear at the end of the motor armature engages with the perimeter of a larger gear wheel with teeth that are roughly trapezoidal in profile. This larger gear wheel is mounted on a short stub shaft that runs in a bearing supported by the gearbox housing, and the stub projects on the outside of the housing to carry the blade itself. The large gear wheel is an interference fit on the stub shaft and was probably heat-shrunk on to it during the manufacturing stage. It is this ‘interference’ connection that has failed, and any slight resistance to the spinning of the blade causes the connection to break down. The Black and Decker replacement part, [which comprises the gear wheel, bearing, stub shaft and housing as one unit] has been ‘unobtanium’ for a long time and I have missed out on a number of attempts to buy a ‘spares or repair’ saw with a duff motor but good mechanicals on Internet Auction sites. I have dismantled the whole thing and there are some signs of scoring on the surface of the stub shaft so it is unlikely that the full transfer of torque will ever be possible without giving the joint some additional mechanical help. There are proprietary ‘bearing assembly compounds’ available that purport to improve the fit of bearings on shafts but I don’t know that they are intended/capable of carrying the full shear stresses across the shaft to gear wheel junction. My only ideas have been to try and file/grind a small slot in the stub shaft and the bore of the gear wheel and to then insert a home-made ‘Woodruff’ key, or alternatively to try and drill a small diameter hole through the gearwheel into the stub shaft and then to lock them together with a steel pin. I have a reasonable drill press and a vice that would hold things steady but I doubt that my normal high speed steel twist drills will make any headway in the gear wheel or stub shaft that are both likely to be of hardened steel. I would be grateful for any advice Jim M0GJD

24th Aug 2022, 11:35 pm	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,880	Re: Black and Decker Circular Saw Without seeing the thing, commenting is difficult. Bearing assembly compounds usually break up if subjected to intermittent forces over a long period. Perhaps a couple of tack welds could be more robust? MIG or TIG would be fast and not too heating. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

25th Aug 2022, 9:54 am	#3
G6ONEDave Octode Join Date: Dec 2019 Location: Owston Ferry, North Lincolnshire, UK. Posts: 1,701	Re: Black and Decker Circular Saw Can't remember the type number but there is a Loctite anaerobic adhesive that will hold under rotational torque, it's a filler type. I've used it on gears that were an interference fit on the end of motor shafts previously. Dave __________________ Quote "All is hyperthetical, until it isn't!" (President Laura Roslin, Battlestar Galactica)

25th Aug 2022, 10:13 am	#4
thermion Tetrode Join Date: Dec 2011 Location: Cheshire, UK. Posts: 89	Re: Black and Decker Circular Saw Loctite 648 bearing retainer is good for things such as this. Another way is to knurl the shaft & press the gear back on. Assuming the shaft isn’t hardened that is

25th Aug 2022, 2:41 pm	#5
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: Black and Decker Circular Saw Studlock is the best. Locktite used to call it 270 but now most of the types with 270 in the number are alternative brands. The bottle I have got at the moment is called Bondloc B270. Try someone like Screwfix. Obviously clean the parts thoroughly with oil free solvent before you even think about using it.

25th Aug 2022, 2:45 pm	#6
Sinewave Octode Join Date: May 2016 Location: Oxfordshire/Bucks borders, UK. Posts: 1,604	Re: Black and Decker Circular Saw Loctite retaining compound or bearing retainer, it's cutting edge stuff for things like this. __________________ Avometer, vintage Fluke and Marconi collector. Also interested in vintage Yaesu and KW.

30th Aug 2022, 9:11 pm	#7
Jimbo Pentode Join Date: Sep 2006 Location: Crewkerne, Somerset, UK. Posts: 114	Re: Black and Decker Circular Saw Thank you for your helpful comments. I opted for the Loctite 648 solution as the shape and hardness of the shaft and gear wheel precluded any 'mechanical' fix that I could attempt in a home workshop. I am impressed with the Loctite product! It's funny stuff.... a small globule on a plastic dish on the workbench was still a gooey liquid after 24 hours, but as soon as it is confined between two surfaces it cures and hardens quite rapidly. A few drops on an M6 thread, [which is a much 'looser' contact fit than a bearing on a shaft], bonded so well that it took a large pair of grips and all my strength to shift it after 24 hours. The saw is now returned to working order and performed a 25mm deep cut in timber with no problem. Thank you again. Moderators, this thread can now be closed, thank you. Jimbo M0GJD