Defective Gearbox/Crank assembly

Well!

I think Brad is on the same assignment as the mysterious Chris Beck so at least we know what the drawings look like.

What we don't know is what the assignment is for, and therefore whether we would be doing someones homework (or their job!).

I don't mind helping with homework but I think if this thread is to continue then the OP should have the grace to explain why he is asking the question.

Ian

Holy smokes! someone dreamed up a real clanger with that assembly. Where do we start? There's a lot more than 30 things wrong that I see....

A good general technique to figure out whether an assembly will even go together is to make paper tracings of the parts and slide them past each other on the assembly to see if they clear. Old fashioned, yes, but cheap and it works.

Another hint is that with rotating machinery for power transmission, each part attached to a shaft needs to be located (usually with a male/female cylinder to cylinder or cone to cone fit) and prevented from rotating relative to each other some way, like keys or dowels or ribs in slots.

It wouldn't hurt to read up on stress / strength in flywheels either - machinery's handbook has a good section about that.

It would be nice if our picture poster and the OP told us what class this is for and why they are asking the forum to chime in.

JD

Thanks Brad,

I was expecting super duper CAD drawings - I guess this quiz has been handed down through generations!

I can see parts poorly located, wrongly fitted, over-sized, under-sized. There are ill-proportioned fittings and poorly designed joints, challenging machining operations and just plain bad design.

With the warning that I'm not a trained engineer, iI suggest you ask yourself:

• How does each of the parts move - will they clash or can they shift from their designed position?
• Are bearings correctly chosen and oriented for their designed loads.
• How would you machine each part - are there things getting in the way that cold be designed out?
• Look at the proportions of parts - is anythhing oversized or undersized relative to other parts carrying similar loads.
• Where would stress concentrate in the design - are there sharp internal corners or thin sections in these places?
• What makes an efficient flywheel?
• Could anything be changed to make assembly easier?
• How are parts mated together - do they have reliable secure joints where fixings just keep them secure, or do the fixings carry the loads?

Go through that list and you should be getting near your 20 things.

Neil

Sorry for the lack of response I've had a few exams to prepare for but now I can get back to the assignment. A few people wanted to know what this assignment is actually for: it's a university assignment worth a small percentage of our overall grade.

Thanks for the replies so far. I think my problem is that I have had no experience with CAD or with gearboxes/crankshafts and so I am having trouble determining what alot of the parts even are which makes it difficult to judge if the parts should/shouldn't be there or are over/under sized etc (this is definitely not my speciality).

Hi Chris Beck.

Here is a few

Casting design

Uneven wall thickness castings with sharp corners and a large mass in the centre of the drawing. It will be difficult to cast; likely to develop "Hot spots" causing shrinkage voids and cracks. ideally castings should be uniform thickness.

Casting design uses three parts with mating surfaces at 90 degrees and in combination, requiring unwarranted low tolerance engineering. Parts should meet 1:1 along single parting line plane.

Insufficient spanner clearance on many fasteners

The gland on the oscillating shaft is impossible to assemble. it is enclosed by the casting.

Flywheel has mass in the centre and reduced mass at the periphery the exact opposite of good design. the mass must be at the edge. Excessive use of material will result.

Reverse taper on Flywheel? very difficult assembly

Bolt near flywheel will rub and difficult to assemble or is it impossible to assemble?

Bearing near flywheel excessively recessed unneeded bending of main shaft

Main shaft is not securely located axially, it will move up and down causing rubbing and failure.

Bevel gear on Woodruff key not retained it will disengage.

Undersize square drive on crank plate likely to fail. Also difficult to machine, splines would be better.

cantilevered connecting rod excessively far from the crank face on the drive crank pin... Pin too long and too thin More stress

Connecting rod interferes with slider

The welded connecting rod appears undersize?

The gudgeon pin is not retained if it moves it will scrape the sides of the slider bearing surface

The slider crosses a parting line in the casting Bad design. very difficult to retain tolerance

The slider inner bearing surfaces are very difficult to machine requiring special tooling

Bolt on pinion and pulley Terrible design. Will not work. The Fastener will just undo likely causing serious damage if under power.

The pinion shaft is not retained by the bearings it will move axially.

Both pinion shaft bearings are assembled the wrong way.

The inner pinion shaft bearing is not retained

The outer pinion shaft bearing retention is unclear?

The pinion shaft can only be assembled if the main shaft is removed Bad design for assembly.

The above should get you started!

Cheers

John

Edited By John McNamara on 17/04/2013 14:35:45

What is the type of engine called he has shown diagrams for, and are there any diagrams labelling what each part is called? A reply asap would be of great help thanks, I have been given this assignment also.

In particular it would be helpful where these parts are located, and what purpose they serve (I have absolutely no autmotive engineering experience and have been asked to complete this by the 31st):

1. Oscillating Shaft and its' gland
2. Slider (and its' inner bearing)
3. Bevel gear and woodruff key
4. Square drive in crank plate
5. Gudgeon pin
6. Cantilevered connecting rod
7. Pinion shaft

I can see this thread attracting desparate students for years.

It isn't a real item, so the parts don't necessarily have proper 'functions', although some could be redundant or unable to work the way they should.

There's no absolute right or wrong. It's a test of whether or not you can visualise a drawing in three dimensions, and make reasoned judgements on poor points of the design. Can you imagine how it would actually be put together well enough to identify where assembley would be difficult or impossible? Can you see how it would operate well enough to see problems? Have you the knowledge to identify refatures that would be difficult to cast or machine?

It's an extraordinarly useful exercise to do properly, and if anyone doesn't enjoy the challenge sitting down with it for an hour or two and puzzling it out, maybe they aren't cut out for engineering?

Neil

Good point Neil. Students should do the research and work themselves, there is a lot to learn in the exercise. If they don't have sufficient background to do it, they should ask the prof to teach some background. Surely eng students can noodle out at least how parts assemble and disassemble (or not) with bits of paper slid over each other, at least. Visualizing that and thinking/visualizing how forces might be acting in a mechanism is crucial in mechanical design, so get started, students!

JD

If these students aren't being taught any basic eng. ( as seems so ) would it be a bad idea for them to get hold of an old i/c engine and take it to bits ? There are loads about for scrap price.

I appreciate your guys opinions on us should being able to visualise the piece but i'm not sure you realise that the majority of practical work in engineering in British universities is done in the finals project, with all the run-up to that being CAD work, technological science, modelling analysis, materials science and business/economics. This work has clearly been taken from a time in the course before such modules existed, when practical work was far more in focus, which is why so many students have found this post helpful.

Thanks Fergal,

I would have thought that engineering students would be champing at the bit for the opportunity to cut metal!

I can't help thinking that an afternoon a week in a workshop during the first year would make everything else easier. At the very least, hand machining and fitting will teach you much about the stresses invoved in machining and the properties of materials.

Neil