Advice wanted on steel gauge

[David perry 1]

Almost, but not quite, modelling.

 

I have built a large Dobsonian telescope, so large in fact that I cannot physically move it. ha ha, laughs on me, BUT it was always destined to be wheelable.

 

The thing weighs about 50lb or so and what I want is a wheelbarrow style set of handles for it.  I have thought of making them from oblong section steel tube but I'm not sure on the gauges / sizes that I'll need. Im thinking of 1cm x 3cm tube (if that can be bought) and about 1.5mm wall.

 

Will that lift 50lb without deforming? 

 

D 

[Ernie]

Hi David, easily, I guess, assuming the 3cm is up and down, and the design is good

How are you going to join the tubes?

 

ernie

[David perry 1]

Front axle is a 25mm steel tube with B and Q green wheelbarrow wheels.  The handles will be joined where they cant with a sliding fit steel plug, a bit like wing joiners in a glider.

 

yes the 3cm is up / down as the section will bolt flat to the large box assy of the telescope.

 

D 

[Biggles' Elder Brother - Moderator]

David, this is not clear cut. Doing a quick rough back of the envelope calc reveals all!

 

First a couple of simplifing assumptions:

 

1. Your steel tube has acircular cross-section of diameter 3cm - I know you imply its oval but this simplifies that maths a lot if we say its circular.

 

2. You say the load is 50 pounds - that's about 200Newtons and lets say that the arms are 1metre long.

 

3. The wall thickness is 1.5mm.

 

So the max bending stress on the arm is:

 

Stress=Mxr/I

 

where M is the bending moment - equals 200Nm

 

r is the out radius of the tube - equals 1.5cm

 

I is the second moment of area of the tube - equals pi(D0^4-Di^4)/64 so thats 1.3x10-8

 

This means that max stress is approximately 230MPa. That's between a quarter and a tenth of the ultimate tensile strength of steel (depending on the exact make up of the steel). At that level of stress you'd get significant deflection - but it would easily support the load without failure

 

Now your section is taller than it is wide (if I understand you correctly) - so that would help a bit as it would increase the value of "I", and your lever arm may in practice be less than 1 metre long - so that reduces M. So the actual stress would be a bit lower than this calc - say 200MPa.

 

So it comes down to whether you just wish to support the load - in which case its probably OK - or if you need to minimise the deflection whilst supporting the load - in which case its probably not OK and I'd go for a greater wall thickness.

 

BEB

[David perry 1]

BEB

Thanks for that.  I will increase the wall thickness then as I really want it to be sturdy -  the mirrors are very expensive and I'd rather not have any bending / bouncing motion at all.

 

I will try 3mm walls!

 

Thanks for the help.

 

David 

[Phil Claridge]

fly closer david, and you wo'nt need the scope

[Tim Mackey]

Glad to see that professor BEBs "back of fag packet quick calcs" are as simple as ever

[kc]

I am going to suggest it might be easier and cheaper to find some suitable 'sack' barrow ( Black & Decker  used to make one ) or similar  available thing to adapt.  You would have a thing of known strength and welds which are proven.  Wheels, handgrips etc all included.

In any case it would show what size tubes are used normally.  Its the bit where the bolts go through that would fail if my lawnmower handle is anything to go by! .  Perhaps  an ex lawnmower handle would make a starting point too,,,,

Edited By kc on 14/09/2010 15:18:01