Hi Geoffrey,

there's more info in this topic:

Versalab M3 grinder owners

this beaing the most important part (straight from Laura at Versalab):

-- The shaft of the grinder is held quite rigidly in a pair of ball bearings. A perfectly trouble free arrangement that had problems only in the first few grinders. [Mr.Schulman's grinder is serial number 9.] We made a rather tiny change (.010") and all has been quite perfect since.

Still, the shaft is quite long and I'm amazed there are no problems with the shaft wobble, especially since those are ball bearings. I'm curious how far away from the burr the bearings have to be seated in order to eliminate or almost eliminate shaft wobble problems. If I can for example put two HQ roller bearings, 1" from each other on the top of the shaft and that will work I'll be interested in that idea.

Regards,
dsc.

dsc,

We are in agreement with the design issues of the long shaft. However, it appears Versalab has solved the problem:

"The shaft held perfectly in a pair of roller bearings"

Not surprising, but how they are configured?

If there is room, a second bearing could simplify some aspects of the design. And it has the potential to increase the precision between the burrs. But everything is a tradeoff. For example if there is a second bearing above the belt pulley then replacing the belt becomes more complicated. Alternately, placing the bearing lower on the shaft could impede the bean path. However, it could also mean that a smaller diameter shaft may be used which could offset this restriction. Speaking of shafts, I am also curious as to why the shaft diameter is so large on the Versalab grinder. Bearing configuration maybe?

One thing that has not yet been discussed - if it has my apologies for overlooking it - is designing for the worst case. For example, a stone or something hard jamming the burrs! A smooth belt could be a simple solution by allowing the motor pulley to slip. A less costly alternative to "fail safe" device on the motor? And maybe why Versalab says they will not use notched drive belts?

Geoffrey

Or in the motor drive/timer circuit you could place a current sense resistor and if the burrs stopped it could sense the current spike into the motor windings and shut the grinder down. You would have to filter for high current transient pulses in bean breaking.
-c

GB wrote:If there is room, a second bearing could simplify some aspects of the design. And it has the potential to increase the precision between the burrs. But everything is a tradeoff. For example if there is a second bearing above the belt pulley then replacing the belt becomes more complicated.

Not necessary, if you design the upper bearing holder like an arm that extends out between the pulleys. That way you could change the belt by pulling it of the motor, and then sliding it of the shaft pulley. Then sling the new belt on the pulley, over the top bearing and mount and then over the motor pulley. A belt tensioner would also help, like the ones on pillar drills. (you know, hinge the motor on springs with a set screw to put tension on the belt. When you want to change the belt you just screw out the set screw and the belt goes slack.) Another even better way is to do what VW did on the beetle engines, design the belt tension device into the small pulley. The basic idea is that you make the pulley in two pieces, both with slanted sides so they form a \/. Then you set the tension with shims between the two halves, like \.../. This also really simplifies belt changes, as you can just split the pulley, which gives a lot of slack (think going from 100mm pulley to 25mm shaft.)

Are you arguing for a more complex design over a 5 cent sense resistor? A tensioner arm there would be overkill.

Why?

Hi guys,

Geoffrey where did you find the bit about the roller bearings? I thought they were simple ball bearings.

I'm planning to use two top support plates for the bearings, probably 1-2" apart and put the shaft pulley in between them. I agree this will make changing the belt trickier, but then again how often do you change the belt? once every few years? Hopefully such a design will be good enough to support the shaft and make sure it spins properly. Regarding the shaft size I don't know why it's so thick, perhaps to increase stiffness? it actually transitions into a smaller diameter just under the bell-shaped element in the bean 'hopper' if you look at the photos in the Versalab topic. This is of course to allow beans through into the burr chamber, otherwise nothing would come out of the grinder as the thick shaft would block the entrance.

Two things to think about:

- what to use under the burrs for grinds transportation. I was thinking of going with a funnel like on the Versalab, but I'm not sure how the conical burr dispersion pattern is. I'm also not sure if I like the volcano distribution that the funnel produces.

- bean hopper construction. Does it need to be funnel shaped, with a narrow bit at the bottom? All the hoppers I've seen are made like that and I was curious if it's a must or simply something that Mazzer used and others copied?

I will upload some drawings today of what's hopefully is going to be the final design and will start looking for the materials to put this together.

Regards,
dsc.

CRCasey wrote:Are you arguing for a more complex design over a 5 cent sense resistor? A tensioner arm there would be overkill.

Why?

No, I was just throwing out ideas, pick whatever you like. My thoughts didn't even address the needs for something that stops the motor from burning out if the burrs gets lodged, that's another problem, what I'm talking about is how to design the upper bearing carrier so it does not interfere with belt changes. The belt tensioner was just a way of getting the belt taught enough for proper operation while still allowing for easy belt changes. Personally I would go with the split drive pulley, makes easy belt changes, easy setting of the belt tension, and the design is bulletproof up to about 6500 rpm if you use the stamped steel design Volkswagen used, even more if you machine it out of steel or whatever. IF the VW size suits the design, these pulleys are VERY available.

I need to make a drawing of the idea for the upper bearing arm, sadly I don't know any of the tools you use, and Autocad takes too damn long, so wait for the pencil on paper version.

I think the hopper design on most grinders are a product of conventional design, and because it's an easy to make design, but it's not the only way to do it. One thing you might need to pay attention to is the intended bean column over the burrs, this seems to be a product of the burr design, as different grinders have different hopper design to limit the columns height (the round plastic disc in most hoppers).

Whoops, sorry about the typo. I should have typed BALL BEARINGS. And Stuggi is correct, an upper bearing could be designed so that the belt can easily be changed etc. The main point I was trying to convey was to keep the design simple. The Versalab grinder is an almost perfect example.

I still think the upper section of the Versalab grinder shaft is large for a reason(s) that we as yet are unable to determine. Extra stiffness, maybe. But it appears to be about 2 cm in diameter! That is a lot of material. Extra material and turning the cone at the bottom all costs money.

Getting the ideal, slightly mounded distribution of grinds into the porta filter could be a really tricky problem and may be something you will have to solve empirically. However, with conical burrs the grounds will exit with a smaller diameter than with the Versalabs flat burrs and it may be less of an issue.

As for a funnel shaped bean hopper, apart from the throat diameter and slope of the sides it is a very simple solution to the problem of storing beans and getting them into the grinder. Making it removable for cleaning may not be a bad idea?

A interesting project and a great collective.
Geoffrey

And the beat goes on! Love the discussion.

Regarding the lateral forces of the pulley on the drive shaft, I believe Versalab handles it with a pair of stacked plain jane bearings seated in the top and bottom of the upper plate. They handle the vertical thrust caused by the grist by simply seating the pulley against the upper bearing if I discern the imagery correctly. Honestly I don't know how much thrust their burr arrangement generates, but my experience with the Robur is that it is surprisingly little.

More importantly, despite my expectation that the lateral forces imposed by the grist against the long arm of the driveshaft should be substantial, my experience with my little hand grinder tells me otherwise. I truly am willing to trust in the self centering nature of a conic burr system to handle itself.

That said, the lower support bearing as is evolving in this thread has other advantages. IMHO, it helps more to register the vertical position than anything.

I've sketched up, yet another friggin method of vertically adjusting the outer burr support carriage. This one was under the motivation to get the adjusting mechanism away from the top of the device's moving parts. The sliding wedges afford another opportunity for decreasing the vertical distance traveled per revolution of the adjuster. Plus, and this is applicable to a number of the iterations I've thrown up here, I've added a kerf to the bottom of the vertical plate that rides in the slot as an opportunity to simply spring load the plate against the adjustment mechanism. KISS!

edgar

dsc, I just learned another simpler trick for centering on circles that seems to work pretty consistently: first hover over one of the endpoints of an existing circle or the midpoint of a rectangle, then track along the vertex perpendicular to that point in the direction of your intended center. As long as it shows the implied dotted line (and you can hold shift while it does to lock that) you can much more consistently snap to the center of a rectangle or circle!