A quiz for rcm

A quiz:

If you are a brand-name (say like "Barbie-less") for a previously large chain store... one where America previously shopped; and you sell dishwashers....

And said dishwasher has a center twirling arm to spray water around... and the arm is secured with a large plastic screw with big shoulder.

Now, you can put the jets on the arm two ways: one rotates the arm so that the screw, if anything, tightens itself...

And the other so the screw LOOSENS itself....so it backs out, falls out, as does the arm....

So which way would you design it to rotate? Why?

Justify your response. 10 points. Responses quoting Feynman get extra credit.

Question 2: Suppose your firm choose badly. How would solve it in the field?

a) Epoxy screw into place

b) Throw arm away, with screw; tell customer it's an upgrade.

c) Figure out how to get screw to stop doing that.

Reply to
David Lesher
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And hopefully, is prevented from doing so by the shoulder itself.

What kind of service reputation is the firm looking for? Like Maytag's with the service people dying of boredom, or shooting for quick response and short time to repair?

That eliminates the ability to service it in the future. (Assuming that it is possible to get an epoxy to bond to the plastic in question.)

Making for much poorer cleaning.

In order of likely cost to the company:

1) Is it possible to put the arm onto its mount the other side down so it screws the shoulder screw back in? 2) Chew some bubble gum, and apply it to the screw threads before re-installing. 3) Pin the screw with A nylon screw drilled and tapped through the block serving as the "nut" and into the threads of the shoulder screw in question. Since it is presumably hollow, and cross drilled to allow water to flow up into the arm, this means that the nylon screw probably should be of a rather precise length. 4) Send out instructions on how to plug the existing spray holes and drill new ones to cause the rotation to reverse. (This requires service personel with the ability to do this well. Perhaps the company would send out a fixture for this to each service center.) 5) (Most expensive) -- send out new replacement arms which rotate the other direction.

Enjoy, DoN.

Reply to
DoN. Nichols

Q1 rotate to tighten but screw to have shoulder to prevent overtightening Q2 Locktite

Reply to
Bill Noble

Well, given that a few minutes thought at the design stage would have made sure this was not an issue...

yep...

But fastest....

No. First of all, the hole in the bottom of the arm is much larger diameter than the upper one where the screw goes:

--^----^----^--------- ------^---^---^------- | | | o |

-------------------- --------------------

| | / \ ==================== == hollow arm || | ==========================/

.......................^ screw theads down into the center tower

Second, the top has the holes that spray water up.

I've been thinking of that, or slightly melting the screw threads to increase the friction.

The water flows around the female-threaded tower inside the 90. See above.

Actually, that's far cheaper than anything with field labor. I'm thinking of doing the drilling to the existing arm.

The other choice is safety wire. The screw has a ~0.5in tall vertical tab for finger tightening. I could drill it, & run stainless safety wire to the rear of the DW cabinet. Still a PITA, one unnecessary if only the original designers had given it some thought.

Reply to
David Lesher

Well, in no particular order.

1.Look in the bottom of said machine, see if wavy washer has fallen off. 2.Write letter to manufacturer, congratulating them on installing pump with enough power to loosen screws 3.See if such a complex dilemma is covered in the instruction manual. After all, these screws are designed to be periodically removed so you can clean the gunk out....
  1. Who is Feynman?

Andrew VK3BFA.

Reply to
Andrew VK3BFA

-100 points for you!

David

Reply to
David R.Birch

On Sun, 20 Sep 2009 02:57:28 +0000 (UTC), the infamous David Lesher scrawled the following:

See logical 1&3 answer below.

"On the infrequent occasions when I have been called upon in a formal place to play the bongo drums, the introducer never seems to find it necessary to mention that I also do theoretical physics." --RF

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My LPEIIWCCI (underpaid injuneerz, see & below) say to use a Loctite on the screws. (It can only be removed by heat and water, things you'll be unlikely to encounter in a dishwashing scenario. ;)

(Dilbert, is that you?)

Bingo!

1) Design a longer screw which self-tightens (maybe one with a shoulder which bottoms out, leaving the arm free to spin.)

OR

2) Design a locking shoulder into the base which allows a setscrew to hold the plastic screw in place without falling out.

AND

3) Stop subbing out my engineering work to the very lowest paid engineer in India's worst Call Center. They're biased with British ways, the people who also gave us Lucas, Prince of Darkness products.

-- The scientific theory I like best is that the rings of Saturn are composed entirely of lost airline luggage. --Mark Russell

Reply to
Larry Jaques

On Sun, 20 Sep 2009 04:59:30 -0500, the infamous "David R.Birch" scrawled the following:

I'll see you and raise you one:

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"Physics is like sex. Sure, it may give some practical results, but that's not why we do it." --RF

-- The scientific theory I like best is that the rings of Saturn are composed entirely of lost airline luggage. --Mark Russell

Reply to
Larry Jaques

Well, quantum physics depends on probability theory - so, if you can find a Nobel Prize winning physicist, he should be able to calculate the likelihood of the cat escaping from the sealed dishwasher before it drowns and solving the problem....

Andrew VK3BFA.

Did you hear about the Quantum Computer? It works...sometimes.

BTW - thanks for the reference, interesting chap.

Reply to
Andrew VK3BFA

More importantly; read up on Feynman and the sprinkler problem.....

Reply to
David Lesher

In a direction such that the fastener would tend to tighten. Why? Because is is obvious.

'The worthwhile problems are the ones you can really solve or help solve, the ones you can really contribute something to."

C) Can you put a small oring on shoulder screw to give shoulder added friction?

Wes

Reply to
Wes

solve it in the

Perhaps an external-tooth lock washer would work? Search

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for an external-tooth lock washer for an example. phil

Reply to
Phil Kangas

I thought they were just billions of "checks in the mail" which never seemed to get delivered.

Jeff

Reply to
jeff_wisnia

Take plastic screw into shop. Turn new stainless screw out of bar stock. In stainless screw install a nylon insert OR cut shoulder so that a star style lock washer can be used.

Second approach. Pull the bottom arm and ream out the threads, press in a new insert with the opposite threads. Then make a new screw with the proper threads.

Third approach. Turn a stud with the proper thread and a shoulder. Cut threads on top in the opposite thread. Use good locktite to install the stud. Then install the proper nut on top.

Reply to
Steve W.

Why is that a problem? When sucking the fluid out, the pressure on the "nozzle" side is reduced, so the pressure on the opposite side _pushes_ the sprinkler arms "in reverse". Probably not as fast as the jets would push it forward, but it seems ovbious that it would turn "in reverse".

It's like sucking an egg into a milk bottle. The milk bottle doesn't "suck" anything - atmospheric pressure _pushes_ the egg into the bottle.

To do this experiment, you need an old-fashioned milk bottle, or any bottle whose mouth diameter is slightly less than the minor diameter of a boiled egg. Light a piece of paper, drop the burning paper into the bottle, and set the egg on top.

Cheers! Rich

Reply to
Rich Grise

WHy is this a problem? The screw will only "tighten itself" until the shoulder bottoms out, which they should have done at the factory.

So, clockwise.

Cheers! Rich

Reply to
Rich Grise

I wouldn't use it for that; I'd use it to improve the seal between the shoulder and the frame.

The screw has to be hollow with a few ports in the shoulder; otherwise, how does the water get to the arm?

Thanks, Rich

Reply to
Rich Grise

If you say so...

But let's see you "push" it back out!

Reply to
cavelamb

Easy. Drop a piece of dry ice in and hold the bottle neck down.

Paul K. Dickman

Reply to
Paul K. Dickman

OK -

Bottle upside down (egg covering mouth of bottle). Blow into bottle, pressurizing the air inside - egg acts as check valve After building up some pressure, remove your mouth from bottle Egg pops right out.

My father showed me this in maybe first grade while we were discussing the workings of a seltzer bottle.

Reply to
rangerssuck

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