Reviving the STH-8BLD-3, or: Yet Another Speed Reducer

[grep]

Back in 2018, I got hold of a STH-8BLD-3.
I used it for a couple of small leather projects, but without a speed reducer, it primarily it saw use for heavy canvas on a theatre gig I was doing at the time. In 2019 I moved across the country and my art/craft/hobby area had to be colocated with the cats' litter boxes for the past few years. I've had plenty of other stuff to do in the meantime, so I've kept my machines bagged until the last of the kitties eventually passed on. Now that I'm able to eradicate the clouds of fine dust from the room, I've started setting things up again.

Regarding speed reduction, I researched the topic on the forums back in 2018, and again recently. After considering others' builds as well as the purpose-made products available, I still chose to go the DIY route.
The factors leading to my DIY decision:

• I felt that I could build a reducer for less than the cost of a commercial product. As it turns out, I can--so long as I'm paying myself in beer.
• I really really really didn't want to cut or drill extra holes in my table. This was really the biggest factor. Every other solution I'd seen required modifying the existing belt hole or was a new top build.
• I hate myself apparently and don't have enough problems

There are likely simpler ways of accomplishing this, but here's what I cobbled together with a band-saw, drill-press, dremel and hand tools.

Maintaining the original angle coming down from the hand-wheel prevented the need to modify the slot, but made for a long stretch. I haven't measured each belt, but the two add up to ~9ft. This worked to apply load to the pillow-blocks in their strongest direction, directly into a leg; though, beefy as they are, I doubt it much matters here. Note, the belt going to the machine head is on in the wrong direction in these photos.

The vertical boards, and the horizontal bit inside the cross-brace (pictured further below) are all 1x4" poplar.
The slots for the bearing mounts were done on the drill press by setting a fence to slide the board along while drilling a LOT of holes with spade & Forstner bits, then cleaned up with a chisel. The bolt heads and spring washers sit in a pocket and can slide up or down.

 

For the motor mount, I plotted out the base on graph paper and made a template to shift it left and back enough to be out of the initial belt line and to not interfere with the original bolt locations. This ended up being a really tight fit for belt adjustment at the end; if I did it again, I'd position the motor a quarter to half an inch forward. This worked out because link belt is forgiving, and I designed this to allow for adjustment at the pulleys in addition to the motor. It's two pieces of 1/2" birch ply, with the motor bolt heads countersunk and hidden between the boards & table.

These bearings are adjustable, however, the ones I received were very stiff initially. Even after greasing, I worried I'd break something while trying to get them aligned. I ended up bolting each pillow-block to a 2x4 clamped to my bench and worked them around for a while with a 3ft rod until they loosened up.

For the axle, I cut steel bar down to size & chamfered the ends. Using a dremel and file, between bouts of sciatica, I ground about the ugliest usable flats that I'll never show you for the pulley & bearing set-screws. Because my flats are dodgy, and because both pulleys have key-ways & I'm not using keys, I balled up lengths of electrical solder to fit comfortably in each set-screw hole, which mushed down and engaged nicely. This also prevented the set screws from extending into the empty key-ways where they'd be unsupported & weakest.

Once I got everything tensioned, it all moved nicely by hand and easily penetrated several layers of heavy canvas. I'll get some leather under it later this week and see how it does. I suspect I may want to move up to an 8" pulley and designed with this in mind, however I may look to a larger handwheel first if I still find it too fast.

 

Edited November 6, 2023 by grep
clarification

[CowboyBob]

Hey,looks pretty good.The easiest thing to do is get a larger wheel on the reducer so you have at least a 4-1 reduction,5-1 would even be better.Larger handwheels this shaft size are hard to find.

[grep]

11 minutes ago, CowboyBob said:

Hey,looks pretty good.The easiest thing to do is get a larger wheel on the reducer so you have at least a 4-1 reduction,5-1 would even be better.Larger handwheels this shaft size are hard to find.

Thanks! I suspected that might be the case when I couldn't find the shaft diameter published anywhere. Wasn't too keen to remove mine just yet to get a micrometer on it. Will keep an eye out for an 8-10" pulley instead.

[dikman]

Nice job - and no welding required! The simplest way to slow it down even further is to replace the handwheel with a larger pulley BUT you'll probably have to extend the slot in the table where the belt goes through.

[grep]

Curiosity got the best of me and I pulled the hand wheel to measure the shaft.
Looks like ~12.48mm according to a micrometer and a dial caliper; however, both being from harbor-freight and converted from imperial, let's say 12.5mm.
You're both correct, not a standard pulley bore size and the table slot would need extending.

Ran a couple layers of rather stiff 8oz through comfortably at ~80spm--which is already much more controllable that it was.
Then I saw how it starts to bog down if attempting to go much slower. Though the motor may be capable of running under 300rpm, I'll consider 300 the minimum speed to actually sew.

I was able to source a 9" pulley for my reducer shaft; that'll arrive next week. According to my calculations, it should put my lowest useful speed at around 44spm--plenty for my needs.
Will update later.

[dikman]

12.5mm shaft, so it's actually a 1/2" imperial shaft.

[shelr1]

Nice job,  I enjoy seeing the variety of designs for speed reducers.

[CowboyBob]

On 11/7/2023 at 8:02 PM, grep said:

Curiosity got the best of me and I pulled the hand wheel to measure the shaft.
Looks like ~12.48mm according to a micrometer and a dial caliper; however, both being from harbor-freight and converted from imperial, let's say 12.5mm.
You're both correct, not a standard pulley bore size and the table slot would need extending.

Ran a couple layers of rather stiff 8oz through comfortably at ~80spm--which is already much more controllable that it was.
Then I saw how it starts to bog down if attempting to go much slower. Though the motor may be capable of running under 300rpm, I'll consider 300 the minimum speed to actually sew.

I was able to source a 9" pulley for my reducer shaft; that'll arrive next week. According to my calculations, it should put my lowest useful speed at around 44spm--plenty for my needs.
Will update later.

If you turn the knob down lower than 660 it will lose power BUT will go slow @ 660 or above you just need to push lightly with your foot & the should go down to 100 rpm as long as the belts aren't too tight.

Edited November 9, 2023 by CowboyBob

[grep]

On 11/8/2023 at 2:09 PM, dikman said:

12.5mm shaft, so it's actually a 1/2" imperial shaft.

Indeed--sleepy me. As it turns out, they're not quite so inaccurate; the shaft is noticeably shy of 1/2" on every rule I own.

On 11/9/2023 at 8:38 AM, CowboyBob said:

If you turn the knob down lower than 660 it will lose power BUT will go slow @ 660 or above you just need to push lightly with your foot & the should go down to 100 rpm as long as the belts aren't too tight.

Pretty sure they were too tight. Managed to find more of a sweet spot when I aligned for the larger pulley.

 

New pulley arrived, very happy.
Ended up making a new shaft, as I didn't like how well nothing stayed put on the first one. Since the new pulley has a single grub screw and keyway, I decided I'd go ahead and try making keyways for both pulleys, by hand, with a cutoff disk on a dremel. They didn't turn out super pretty--I'm sure the machinists are cringing--but they're the right size and nothing shifts. Also drilled divots for all the non-keyed grub screws, and remembered the loctite this time. That all seems to have done the trick.
Now my machine happily creeps along right down to about half a stitch a second.

[dikman]

Can't go wrong with a nice big pulley! While you might lose top speed to me it's worth it for the slow speed control when sewing leather. Good job.