10K drum gap issue
Hello fellow roasters, wanted to see if anyone had any experience with an issue we're having regularly on our 10K machine.
PROBLEM: Drum gap has to be adjusted 1-3 times over the course of a roast, otherwise beans get dropped through the gap (too wide) OR get jammed in the gap (too small), causing the drum to stall out. Audible grinding sound as the gap closes over time+heat, at which point we have to adjust the gap to continue roasting.
BATCH SIZE: We typically roast only full 10kg batches on this machine. In the past we've done 5kg batches but our current approach to eliminating variables on each machine (as well as the ongoing demand for fulfillment) has us roasting full batches.
SETTINGS: Our base settings for airflow (17/20/23 Hz), gas (2,200 Pa start, down to 1,200 Pa most roasts) and drum (57 Hz) are more or less identical to our initial seasoning/setup roasts with Mill City from 16 months ago. We typically warm up the machine at 1,200 Pa to 440 ET (allow about 30-45 minutes) and then allow 5-10 minutes at 2,200-2,400 (our drum gap is set at this point) before charging the drum.
COFFEES: We have about 16 different coffees we roast regularly, different screen sizes, with the smaller ones from Ethiopia or our decaf coffees creating the most havoc. However, all coffees exhibit the same issue with regard to the drum gap.
OBSERVATIONS: Monitoring IT seems to be one variable involved in this, obviously the drum is expanding with heat gain after the initial charge (IT can vary from 500F-750F over the roast curve). We have played around with gas pressure and airflow. If we start with our high air setting and leave all other settings the same, we can roast a full batch without adjustment (however there are obvious implications for having no airflow adjustments during a batch). The airflow is allowing the heat to remain more consistent on the drum/IT during roast. Alternatively, we have tried shifting our air L/M/H standards upward; however the higher settings can allow a few beans to get sucked out during the roast as well, so this isn't a complete solution.
We have maintained the grease on the front and rear drum bearings, so that is unlikely to be a culprit. The drum itself does appear to have a small amount of visible imperfection (undulation) during rotation, which causes uneven rubbing/grinding sounds when the gap closes too much (it has a rhythmic/pulsing effect to the contact with the front wall). We addressed this on a video call with Aziz about 9 months ago and he assured us it wasn't a concern or was at least within margin of error as far as the actual drum is concerned.
One thought we had was that the machine, being currently level, could benefit from adjusting the feet to tilt slightly back. The logic here is that perhaps the drum is causing too much of the bean mass to be forced forward, leaving too much unbalanced weight at the front. However, that still leads me to think there is some other issue since un-levelling a machine is probably not a long term solution.
Any thoughts would help, going to troubleshoot this a bit longer then probably setup another call to support.
I'm owner of MCR-10 since Dec 2023
allow me to share my experience on few of your points
for your issue on the gap try to let your roaster on 220 C for around 20 min to 30 min and try to reduce the drum rotation I usually use 57 HZ so I set it to 45 HZ and "keep the gap as large as possible "after that try to reduce the gap slowly till you hear the grinding sound then you turn to clockwise very slowly to let the grinding sound disappear.
in this way, only the chaff will be on the burner chamber.
on batch size, I feel that the sweet spot is 8 KG, not 10 KG
I'm on the MCR-20 with similar warm up protocol and origin variety. I've also had this issue not long ago where it got pretty bad - every now and again beans would load up in the gap and then it'd be a delicate game of trying to close the gap while trying to let the drum cycle the beans down the waste chute without stalling.
These days, it's been better with me only having to make minimal adjustments until the beans hit yellow and then it's stable from there. When charging, I only open the hopper gate about halfway or so to slow down the amount of beans. I think this gives the bean mass enough time to spread out evenly throughout the drum and also reduces amount of heat loss from the front of the drum.
We spoke with support, initial recommendation is to increase charge temp.
We bumped up to 460F and ran a batch of Honduras (pretty easy bean to roast, large screen size). No issues, no gap adjustment.
Next we'll try a smaller screen Ethiopia Guji and see if it handles well.
Overall only very minor adjustments to our profile were required. These large batches don't seem to mind the 35F increase in charge at the same gas and air.
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