Member postings for Ian Austin

I saw that, but I think this is a reference to the concentricity run-out on the taper - possibly an issue in large diameter lower-quality taper-locks, but unlikely to be a problem in the wee little ones we're talking about. There is widespread industrial use now of taperlocks - if they were no good (i.e. maintenance = cost money) they would have disappeared. Also, note in uniformly-loaded low-service factor situations they don't need a key (see Fenner Drives link below), i.e. unless your torque overwhelms the shrink-fit-like grip that the taper exerts on the hub, or you have high dynamic loadings. Mine are for a machine that is still in bits.

Read this Fenner Drives technical guide on how to use them:

https://www.fptgroup.com/dss/docs/689_06_Fenner_Shaft_Fixings.pdf

Edited for better info

Edited By Ian Austin on 08/08/2018 12:10:31

Ha, too easy to "engineer" in front of a computer screen! I know that one!

It just so happened that I bought a couple of taper lock pulleys only last week, so that's why I'm specced up on them. I was really surprised how cheap they were - from the major trade-industrial supplier here too - the one with the usually-scary prices. Two bushes and two pulleys (1210 size) for $40 which is about £23.

Better to have a reamer and broach hanging around than a disused pulley - good plan!

Richard, unfortunately I am locked out of these Def Stans by a paywall. As is the BS2573 and for that matter AS 1418.1 (the local one here). I guess we will just have to take your word for it unless someone has a standard to hand, to check.

In googling I did find this handy snippet:

"In summary

SWL has been phased out and should no longer be used, and all reasonable practicable efforts should be made to replace SWL with MRC.
MRC should be used for all cranes, hoists and winches. The MRC must be clearly labelled on both sides of the crane beam or boom.
WLL should be used for all for all lifting devices below the crane hook. Allowance must be taken into consideration for the arrangement of the lifting devices by derating the WLL."

source: https://asseteng.com.au/do-you-know-the-difference-between-swl-wll-mrc-and-when-they-should-be-used/

Duncan, that's great. So following the BS standard for ultimate factor of safety set at 6, Euler's analysis does a pretty good job of predicting a commercial load rating for a hoisting tripod, after all. That sequence I posted earlier just needs SF=6 and the result is 1400kg. It's a more realistic result. Just need to make one and test it at 125% rated load (is the 125% requirement also BS or just ANSI?) -- if it keeps standing, it's good. Still not quite sure of the correct way to test the 10% side load though.

You seem overlooked that I demonstrated the correct taper lock bush and pulley (i.e. 22mm 'long', to fit the length of your shaft) are available. Just call a good industrial supplier and ensure they understand the limiting factor is the axial length. The products are available. You just need to specify correctly.

Richard, this is good solid info. Can you point me toward a published guideline that mentions these factors? The 'swing factor' could I suppose be modelled by subtracting a further 10 degrees or so from the leg angle (i.e. 'flatter' tripod, less strong). Do you think?

Duncan, yes good point, the risk is that an overloaded or defective tripod -- or one that someone heavily knocks into -- might give way by sudden buckling without the warning signs of previous gradual deflection. So a high SF makes sense.

Dave, looks promising. It would be interesting, when you get it properly set up, to apply a point load midway up to one leg to simulate an eccentric loading, just to see how vulnerable the structure is to being hit either by a swinging load or by someone slightly backing their truck into it.

Look, here you go -

http://www.bearing.net.au/wp-content/uploads/2014/10/Taperlock-bushes.pdf

1008 bush, axial length 7/8" = 22.25mm. Perfect.

EDIT:

Sorry, just realised you've got a single groove pulley (was about to ask why you needed a 2-groove pulley, looked again at your photo).

Not to worry. Have a look at this catalogue below, you'll see you can get SPZ pulleys for 1008 bushes that are also 22mm axial length. Yes these are Australian catalogues but you know where it all comes from, eh? If they're here, they'll be there close to you too.

http://www.bearing.net.au/wp-content/uploads/2014/07/SKF-Pulley-Catalogue.pdf

Edited By Ian Austin on 07/08/2018 13:47:36

Good point, Richard. That greater safety factor does bring the theory much more in line with the practice. Ta.

Dave, I hope you're going to give us screen shots of that FEM analysis! Love to see it - would be very impressive.

I have an SPZ taper lock pulley and bush here in hand.

I see from your photo that your bush is a 1008 11mm bore, and the SPZ pulley is two groove.

Ok, so mine is a little bigger - SPZ 1210 18mm bore, but still a 2 groove SPZ pulley.

The pulley is 28mm axial length, so I assume yours is too?

And the bush - although a bigger size - has an axial length of 25.5mm (with the 2.5mm recess on the motor side). So I would have thought you could get a 1008 bush somewhere that is also 25.5mm long, at the most. But because its more tiddly, maybe you'd even find a 1008 bush that's 22mm or so long.

Try googling around suppliers catalogues to see if you can't get a thinner bush.

Well, I eventually did make a calculation of the MEW-published tripod according to Euler's critical load formula for buckling columns. I'll copy-paste in the whole calculation below, along with a short discussion.

The executive summary is this: I used a spreadsheet, and ensured that the spreadsheet would replicate another example I found on the web -- i.e. the spreadsheet does not contain any silly mistakes. I used the dimensions mentioned in the MEW article. I de-rated the Euler's calculation result by a safety factor of 2. The resulting maximum allowable load -- by Euler's theory, not by field test -- was 4200kg. In contrast, a similarly-sized commercial steel tripod (by Spanco) is rated at 1800kg for the heavy-duty version - with legs that look a lot chunkier than the 48mm scaffold poles here. So I would suggest that the simple Euler's calculation is not the full story.

For those that might like to see the nuts and bolts, here's my working-out and fuller discussion:

* * * * * * *

Tripod rated load – theoretical calculation by Euler’s critical load formula

As the legs on a tripod are slender columns, assume critical stress is lower than yield stress, and assume the failure mode will be buckling, where the column is subjected to a load greater than the tensile strength along the column’s long axis. Load is assumed to be purely axial without any eccentric load, so no bending stresses are calculated. This also implies that the load must not be swung in case it hits one of the legs. It is assumed that the legs are restrained from movement at their base by a lashing rope and at their head by pinned connections to a cap. The load is a suspended between the legs by an eyebolt centrally placed in the cap.

The calculation below relies on finding the critical load leading to failure, using Euler’s formula for critical load. This formula provides a value for the ultimate load, but to be sure that the structure will not fail, an ultimate factor of safety has to be applied, so that the structure will be sure to support the rated load without failing even if there are local imperfections in the materials.

Material: Standard steel scaffold poles

Modulus of Elasticity, E (approx.) = 200 GPa

Outside dia, Do = 48.3 mm

Inside dia, Di = 40.3 mm

Length of legs, L = 3.05 m

Angle of legs to ground = 70 degrees

Ultimate factor of safety (failure load/allowable load), FS = 2

Axial load capacity of one leg:

Area moment of inertia, I = (Do^4 - Di^4)*pi/64

where Do and Di are expressed in metres

I = (0.0483^4-0.0403^4)*pi/64 = 0.000000138 = 1.38*10^-7

Euler’s critical load:

Pcr = pi^2 * E * I / (K * L)^2

where

Pcr = Euler's critical load (longitudinal compression load on column),

E = modulus of elasticity of column material (200 GPa, expressed as 200*10^9 Pa),

I = minimum area moment of inertia of the cross section of the column,

L = unsupported length of column,

K = column effective length factor (take K as 1.0, i.e. both ends pinned)

Pcr = pi^2 * 200*10^9 * 1.37*10-7 / (1 * 3.05)^2 = 29214 N

Calculate the rated axial load capacity, Pax, of one leg:

(Pax is the force directed along the inclined leg)

Pax = Pcr / FS

Pax = axial load capacity of one leg = 29214 / 2 = 14607 N

Calculate Pz, the vertical component of Pax:

Pz = Pax * sin (70) = 13726 N

Calculate the total load for the three legs:

Total load Pztot = 3 * Pz = 41178 N

To obtain maximum permissible load in kilogram:

convert value for force in Newton to kilogram-force, where 1 kgf = 9.807 N

Pztot / 9.807 = 4200 kg

The result: according to the Euler buckling analysis, maximum permissible load on the tripod is 4200kg

This would need to be field tested by suspending a load 125% of rated capacity, i.e. 5250 kg to prove that 4200 kg is indeed the permissible maximum load rating.

The maximum load value changes as the legs are splayed more or less. If legs are splayed further out to a 60 degree angle, the maximum load decreases from 4200 kg to 3870 kg; whereas if the legs are moved in to an 80 degree angle the maximum load increases to 4400 kg.

In reality, commercial tripods in this size have far more conservative maximum load ratings. Spanco, an American company, sells steel tripods with legs that can adjust from 3.09m in length to 4.93m. They make the tripods in two grades, one rated to 1 ton (900kg) maximum load, and the other to 2 ton (1800kg). Incidentally, the Spanco tripod product manual states that the legs should not be splayed out to more than what works out to be about 70 degrees angle.

The comparison of the Euler's analysis to the commercial practice casts doubt on the usefulness of Euler's analysis, at least on its own, as a way to assess a load rating for a hoisting tripod.

* * * * * * *

Wouldn't that only be if the lugs holding the tube sockets bent across the plane of action? As in, two of the lug-pairs would have to bend as the third leg moved over the centre? Or am I not visualising that right?

Re scouts - yes, we made one like that at scouts too -- which has just reminded me of a description of that type (rope and timber poles) in the Admiralty Manual of Seamanship 1952 edition vol2, pp 218-19, as an "extempore gyn" -- I'd forgotten about that, just looking at that now.

Edited By Ian Austin on 06/08/2018 11:34:41

OP ... should have suggested in the first place ... who? moi? Oh, I'm just learning as I go -- knew nothing about any of it before yesterday.

But nice interpretative commentary on the different designs, Clive.

I guess one other thing about the original design published in MEW, with its fixed-angle pole sockets, is that it will really only work on level ground, whereas, to some extent, the swinging leg types might be adjusted to uneven ground -- some designs also have telescoping legs, with a series of holes along them for pins -- nice for sloping ground. Telescoping legs would then have the anti-splay chain halfway up, at the base of the upper tube.

Also, it becomes clearer why the original MEW-published design is less suited to free-for-all home fabrication, because the length of weld that joins the sockets to the plate is tiny in comparison to these other types that have a longer joining surface - making the quality of the weld far more critical in the MEW design, and more redundant in the others. As well, there is the issue of leverage pointed out in an earlier post -- i.e. it is fairly easy to apply a damaging lever force to the sockets on the MEW-published design - a disadvantage that is absent from the 'cap' type designs.

Edited By Ian Austin on 06/08/2018 11:09:26

Edited By Ian Austin on 06/08/2018 11:14:16

Here is a sketch of the scaffold-pole tripod head that appeared in a second-hand advert, mentioned above.

I had a google around for images of 'scaffold pole tripod', 'tripod hoist', 'tripod lifting crane' etc.

Here are just a few of what to me are the most interesting results, showing the tripod head in commercial products of about the same type we have been discussing:

Vestil:
https://www.vestil.com/products/mhequip/tripod_hoist_stands.htm

Cisco-Eagle:
http://www.cisco-eagle.com/catalog/product/166339/aluminum-tripod-gantry-crane-1-ton-cap-8-7-max-lifting-ht

Lifting Safety UK:
https://www.liftingsafety.co.uk/product/aluminium-tripod-crane-1000kg-3090.html

Unknown brand:
https://www.preloved.co.uk/adverts/show/114580540/lifting-tripod-head.html 

To me the first two websites both from USA) show the most interesting design of tripod head. Two different brands (Cisco-Eagle and Vestil), but similar thinking. The Lifting Safety UK design looks for difficult to fabricate but this is a UK forum so I've linked it.

The last image is from a current UK second-hand advert, (the link will die fairly soon so take a screenshot while you can), but is possibly most like what various participants in this thread here might be thinking of, and is also very interesting.

All seem to use a threaded eyebolt for the suspension point. Note also the style of feet on the US designs, and the use of an anti-spreading safety chain or rope, either at mid-height or at the feet.

I think its great that MEW will take a second look at design of this handy type of equipment, and I'll be interested to see what Neil comes up with.

Although I suppose I've been contentious and offputting for a first-time poster, I won't be convinced that the bar for safety on lifting equipment is anything but high, and any diy construction or design discussion needs to reflect that.

Edited By Ian Austin on 06/08/2018 00:25:22

Edited By Ian Austin on 06/08/2018 00:28:21

Thanks Dave, that's the kind of rough estimate I was looking for to see if this design would suit loads to my probable limit of about 500kg, and it would seem that it would do, other factors being acceptable.

The ideas for reconfiguring the plate are welcome. Not being a professional welder I can't assume that my welds are good enough to guarantee safety in all circumstances, so looking for redundancies or natural advantages as in your diagram B, rather than skimpy theoretical minimalism relying on weld strength alone as in A, is a better option for me.

As to personal injury liability, it's definitely moved on from the old days of common sense. It used to be your own fault if you were a dimwit. Now we all have to keep in mind "what if a dimwit got hold of this", regardless of what common sense we might expect people to have. Hence the need for disclaimers and safety statements inserted by editors. But once the disclaimers, warnings and proper use statements are all dutifully done (as in any modern tool operation manual), there's no need to back away from what we like to do.

Some nice comments there -- Martin C has nailed the engineering far, far better then me.

I guess the legal defence is that MEW never specifically endorsed the design for replication -- so it would be up to any injured party (if that were to occur) to argue that by the design's publication the plaintiff took it as an endorsement the design was suitable as a diy project to be replicated (contextualised in a diy magazine after all, full of designs there to be replicated). After all, the article never warned that that was not the case. There was no safety warning, guidelines, discussion of dangers or advice not to try this at home. I'd bet on the injured party winning, but who knows.

One good thing is that the design knocks down to scaffold poles and a strange triangular plate thingy -- rather than being a permanently assembled unit, so it isn't something that would be sold on masquerading as an approved lifting device (intentionally or not) in, for example, an estate sale after the maker's death.

Neil, yes you have a difficult path to negotiate there, because of the home welding wildcard -- I recall when I welded some brackets for a friend, I hadn't hard-tightened the big slider knob for the current, and the buzzing had slowly pulled the knob in, reducing the current. The welds still looked ok, but once I’d done, I realised you could just snap them apart. And then there’s the issue of welding galvanised pipe/plate – if you don’t know to grind off the zinc plating, the weld just peels off later on. And yet, here’s a design for a hoist frame that might have a tonne swinging off it. So yes, lots of factors.

And yet, if you don’t recommend an upper load limit for the design, you’ve got the possibility that someone not tuned in to what they are doing might try to lift something that perhaps this design, even if well-made, really can’t support. At the moment nobody knows what that limit is – it’s just a guess. I would probably say 500kg (if made well, and allowing a safety factor because of dynamic loading) but someone else might say she’ll be right for 1500kg. As it stands, who knows?

A load rating has to be stated, even if you have to pay £300 or whatever for an engineer to sign off on a design (not a product). That’s the burden of publishing. And that engineer would be specifying that the welds, the steel, the dimensions, the hooks, the chain, and so on are all to relevant British Standards. And add to that, that the operator is aware of proper operation. If the specification is done, and the warning is there that that is the deal, then the culpability is probably lowered right down. At the moment, by holding onto the idea that it’s better to stay stumm on these aspects, even though you've now published a design, over here in Australia you’d be a walking lawyer’s target. The way case law has progressed on liability over the last few years, it’s just scary.

If you look at the Australian Model Engineering mag, or the Australian diy websites, everyone is very careful about what they say publicly around anything that should be certified - electricals, boilers etc. Model boilers all have to be built to standards, tested and certified. Electricals hard wired to the supply, or even not, always have some version of a statement to used a qualified electrician. Yes, it might be arse covering but who wants to be a sacrificial bunny? I don’t know about the UK.

Of course, the other way to go on publishing on things like a hoist is not to offer a specific design for fabrication and use, but to simply discuss what’s on the market, especially here in this case, of the ‘scaffold pole’ type (the type in the article) apparently which is also a type sold commercially, mentioned by David George above in connection with roadwork. If some fellow then says in his workshop ‘I could make that’, goes ahead and injures himself or someone else as a result, that’s on him.

All that said, I’m personally happy that the design is there – as a start to my own ideas – but see how already we’re talking about how it’s not a great published design and needs improvement.

You've expressed something that was in the back of my mind about the 'adapter plate' -- that the weld-ons didn't look the full business, especially the hook-eye in the centre which would be under considerable tension and high dynamic loads if anything was swinging or especially if the load suddenly slipped. Yes, a commercial solution there, although I'm still trying to visualise exactly what, would be better than the weedy U-bolt weld-on. And good point about gussets on the pole sockets, just right.

The quality of the welds is beyond MEW's control, but given the history of industrial accidents with lifting equipment, hence the web of design standards, workplace legislation and case law around them, the design ought to have been given a stated rating as part and parcel of publication -- it's the act of publication that raises the level of due diligence. Other than the lawyer's picnic if the design/author/publisher can be upheld as a contributor to injury, its all about safety first.

Two interesting points there about your roads tripod -- the safety chain, and that it was made from scaffold poles (as is the MEW design) with a block rated to 500kg. Which raises the question whether the MEW design ought really be rated at 500kg rather than being assumed to carry 1000kg.

I also searched for any earlier thread on this tripod design before posting but didn't find one either.

I have just had a closer read of an article appearing in Model Engineer's Workshop 264 (February 2018) by Stan Nesbitt on construction of a chain hoist tripod. The design has many advantages I think, in its use of readily available and low-cost parts, its portability and its simplicity. I like it, and intend to make one.

The only thing is, it is a lifting device -- and yet no load rating was declared for the tripod itself (though the load limit of the recommended retail chain hoist block is noted). I am surprised that details on its fabrication were published without that information - surely standards apply to lifting devices in the UK as they do in this part of the world, and publication would seem to be an implied endorsement of the tripod's fitness for purpose. No mention either of limits to usage on slopes or such like. Is MEW not a little circumspect when it comes to liability? Or do you not have a thriving industry of personal injury lawyers like over here (Australia), ready to make the most of anyone connected to a mishap?

Well, anyway, MEW's refreshing boldness aside, could anyone venture a load rating on the tripod as shown in the article?

And secondly, the photos show use in soft ground where the legs presumably dig in to prevent splaying. What would be the situation if used on hard smooth ground such as concrete? Are we sure that under maximum load (whatever that might be), there is no tendency for the feet to slide out and splay? I guess not, but I'm no expert.