Thread: Vibrating DC

Keep your shirt on and things like the won't happen Bob.

I suppose it was a good thing you didn't try to run at 65Hz when you started turning.

Pete

Originally Posted by BobL

BTW This would not worry cyclone users.

Unfortunately that is not the case, I know of two incidents much the same involving cyclones and one blew out the side of the impeller housing (a Men's Shed), the second was in the US just recently and I know in that one the impeller was slightly damaged and I think the wrapping for the impeller suffered also IIRC.

Originally Posted by Chris Parks

Unfortunately that is not the case, I know of two incidents much the same involving cyclones and one blew out the side of the impeller housing (a Men's Shed), the second was in the US just recently and I know in that one the impeller was slightly damaged and I think the wrapping for the impeller suffered also IIRC.

I guess the rag had too much surface area to weight and was carried along in the air currents?

Mine does not seem to have suffered - I'll put the vibration meter on it when I get a chance as I can compare it to when I first installed it.

During a phone conversation with Bill Pentz his thinking was that running a CV cyclone faster than 70hz was dangerous due to the risk of something getting into the impeller housing and what would then happen. I think that was a bit of a broad statement and while high speeds on a heavy impeller might not be advisable for motor reasons I never really agreed with it because I had seen first hand that low speeds did exactly what he was talking about. The only difference was going to be the extent of the damage and in both instances there was going to be major damage anyway so why worry too much about the speed. The one I witnessed the aftermath of happened at 59hz, the impeller housing lost part of the wrapper which we repaired and the motor and impeller are still spining as I write this many years after the incident. I would advise any wood turner to either turn off the DE when using a rag or have a swinging guard on the inlet if they want to keep it running when using a rag to apply a finish.

Also make sure the bin is airtight

I don't think it's necessary to turn off the DC if a mesh guard is used over BMH.

I've tested a 25mm square x 3mm mesh and it only reduces the flow by about 1% and there won't be any large rags getting through that size opening.

LatheBMHguard.JPG

Originally Posted by BobL

I don't think it's necessary to turn off the DC if a mesh guard is used over BMH.

It is one of two options and it is obvious that using a mesh guard makes that option unnecessary.

Originally Posted by Chris Parks

It is one of two options and it is obvious that using a mesh guard makes that option unnecessary.

But it doesn't look very pretty, Bob.

Mesh guard.jpg

Originally Posted by GraemeCook

But it doesn't look very pretty, Bob.

Mesh guard.jpg

Interestingly I tried a cut down versions like this.
latheold.jpg
But, this one reduces the air flow by about 7% and I'd rather Have the extra flow than the tidings.

The external guard can be easily removed when I'm not using rags or parting off two small workpieces

Originally Posted by BobL

Interestingly I tried a cut down versions like this.
latheold.jpg
But, this one reduces the air flow by about 7%...

Interesting enough that I reflexively had to do a sanity check. Seemed only fair to share it on a rainy Saturday morning.

Let's start with the larger loss for better resolution. So the flow was decreased to 0.93 of what is was before the grid was installed. Let's square and invert that to get 1.156. Subtract 1 and we have the added pressure loss divided by the maximum fan pressure, so a relative pressure loss of 0.156 added by the grid.

Assuming a BMH of .25D the area of the intake is about 2.25 times the area of the pipe for a velocity pressure 5 times lower. The delta p is velocity pressure times loss coefficient so instead of the 0.156 inside the pipe we would expect a loss of 0.156/5 when the grid is moved to the entrance. Working back to flow we take the square root of 1.0312 and invert it to get 0.985, close enough to the 1% loss you measured.

Ok, I'm going to make some more coffee and just sit and watch the rain for a while.

The math went over my head but I get the principle as to why having the screen forward works better. Thanks.

Pete

Originally Posted by dmorse

Interesting enough that I reflexively had to do a sanity check. Seemed only fair to share it on a rainy Saturday morning.

Let's start with the larger loss for better resolution. So the flow was decreased to 0.93 of what is was before the grid was installed. Let's square and invert that to get 1.156. Subtract 1 and we have the added pressure loss divided by the maximum fan pressure, so a relative pressure loss of 0.156 added by the grid.

Assuming a BMH of .25D the area of the intake is about 2.25 times the area of the pipe for a velocity pressure 5 times lower. The delta p is velocity pressure times loss coefficient so instead of the 0.156 inside the pipe we would expect a loss of 0.156/5 when the grid is moved to the entrance. Working back to flow we take the square root of 1.0312 and invert it to get 0.985, close enough to the 1% loss you measured.

Ok, I'm going to make some more coffee and just sit and watch the rain for a while.

Hope the rain was worth watching - we've just had our second driest recorded July ever here in Perth. Good weather for solar power though.

My measurements of flow loss were performed on the end of about 4m of test 150 and 225 mm dust but in practice (on my lathe) one should also probably take into consideration that with the guard on the outside of the BMH it is then further away from the hard 1R bend directly behind the BMH. With the guard on the inside of the BMH its almost inside the bend.

Well, it's raining again. We had a dry spell earlier this summer but for the most part rainfall has been about normal here.

I'm sure that elbow has an effect. Most of the flow is probably confined to the top half or so of the intake. Again, I was just doing a sanity check, not trying to hit Mars orbit. I fact, I was happy to just divide 7% by 5 (the square of the area ratio) to get 1.4%. It was only when I decided to post that I felt obligated to get the calculator and do some arithmetic.