RPM of mixing auger/ boot to mixing auger clearance?

RPM of mixing auger/ boot to mixing auger clearance?

Postby JayD on Wed Dec 16, 2009 3:06 pm

Hello: was hoping to find out what is the best RPM to turn a 9 foot by 9 inch mixing screw. Would like to premix for gunite, & regular concrete. Also how tight should I set up boot to mixing auger clearance?
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Re: RPM of mixing auger/ boot to mixing auger clearance?

Postby smnstn on Sat Dec 19, 2009 11:36 am

Dan Paige Sales in Sacramento, CA could be your best source of info re the gunite aspect of things. Google the company name for their website, and note their toll free number. They are not good about returning e-mails, but you can generally talk to somebody on the phone right away, or get a callback in a reasonable length of time.

As for auger speed, that may be somewhat dependent on the make of the unit. Generally speaking, a 9-inch auger should turn at a minimum of 300 rpm. With specialized auger flighting patterns, this may call for variation. The manufacturer of the unit should help you with that. If you hit a dead end because of the economy and layoffs, send me e-mail directly at smnstn@msn.com. There is more resistance mixing true gunite because the sand and stone are relatively dry and the cement combines with any moisture in the sand to make a gummy, hard to move mixture. Unless there is a really extreme reason not to do so, I'd keep my auger speed the same as with regular concrete, to be sure to move the material out of the small, 9-inch tube as quickly as possible, but I would lower my mixing angle which will decrease resistance inside the boot. Most gunite receiving pots are not very tall, and a lowered mix angle is desirable anyway. Shotcrete is a different matter. The terms are used interchangeably these days, and are not the same thing. If you need further information for this, let me know.

Auger/boot clearance should not permit you to insert your fingers fully between the stopped and locked out auger and the rubber of the boot. Your mixer is likely equipped with wear plates which will wear dramatically more quickly on the lower third third of the shaft while those near the top will wear hardly at all. So when you replace plates, you can save some money by replacing only those that need it. Do not allow wear to occur down into the bolt holes. Change them before it gets that bad.

Premixing materials before loading them for production is a great way to maintain consistency, but is very slow and fuel consumptive. Perhaps constructing a third holding bin would help. I this bin, use your front end loader to deposit a bucket of stone, then a bucket of sand, then a bucket of stone, then sand, etc. You can figure your ratio more precisely than that by figuring standard mixes for these machines should be 55% stone to 45% sand. But because of particle size, generally an equal number of scoops will be right. When you hae a few scoops in place, then "roll" the pile with your loader. Doesn't use as much fuel as blending with the truck, and there's no wear on truck and mixer parts. When you load your bins "roll" the pile again by scooping a bucketful from the bottom of the pile, dumping it over the top of the pile and then picking up your bucketful to load from the rolled area. Don't forget to set your gates at 60/40 or 55/45 of your original gate total after calibration with separate materials. If your original calculations called for gate settings of 5.4 and 4.6, your total is 10. With pre-blended aggregate, since your bins are of unequal size, set your large (stone) bin gate at 6 and the other one at 4. If they don;t run out at the same time, adjust them so that they do. For richer or leaner mixes, vary your gate settings by keeping the same relationship between the two gates.

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Re: RPM of mixing auger/ boot to mixing auger clearance?

Postby harberconcrete on Sat Dec 19, 2009 10:27 pm

I know that I am in the minority but I am not convinced that you can get a consistent/accurate blend with a loader bucket, when you produce concrete/gunite/shotcrete etc. by the shovel full (the amount that falls into the mixing chamber/auger each second or so) I believe that your material needs to be consistent!!, also it would seem to me that if you have a blend of 3/8 inch aggregate blended with sand as opposed to 3/8 in one bin and sand in another you will get more material through your gate with the pre blended material don't assume CALIBRATE. SOOO if you produce shotcrete or gunite regularly use your auger to produce the correct blend of fine/coarse aggregate or accept that your 8 yard mixer may only be able to produce 6 yard or so since you will use much more sand than usual if you carry fine/coarse aggregate in seperate bins, or if you operate in an area with liberal DOT weight laws you may be able to build extensions on the sand bin to load more sand (and less rock), ex. California allows only 34,000 lbs on the back tandem axles but I could get 10,000 lbs on one side of each axle (I know it does'nt make since) but check your limits and see what works realizing that carrying a significant amount of weight on one side of your truck also has its detriments.
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