Hi-Lift bending..

[Maxtd5def]

G'day all,
Had to change a peeled tyre on a rock step yesterday. The hi-lift was bowing, it wasn't a good look, although it held.

The rack straightened up again as the load came off. Is it cactus? OK to use again? Its 10 years old, and they're cheap I know, but..

i read somewhere that the rated capacity is only 6" from the bottom, at the top of the rack they're only 10% of the capacity, or something like that. Has anyone got anymore info about the load these things will cope with?

Has anyone had a rack fail?

Regards
Max P

[Tiny]

dont know BUT HiLifts are for assisting to de-bog a bogged car by lifting he wheels to pack stuff under, that is it, they are dangerous and should not be used to change tyres, or lift the car to work on or under in any circumstance, we had to work on a car at appin yesterday so we lifted the wheel, packed under the spring pack and removed the jack. still not exacly the safest option, but a 40cm thick log aint gunna fail like a HiLift

[BundyRumandCoke]

I've had mine flex sideways, and return without any problems.

[Bush65]

Max,

That is normal, all hi-lifts do it.

A 1st year mech eng student could have told the original makers that it would happen (Eurler's column theory) and how it should be changed.

Funnily all of the immitators make minor changes, but they all build in the same shortcoming that gives them such a bad (and deserved) reputation.

[uninformed]

so John how hard would it be to make a better one?
serg

[Slunnie]

Pivot on the base?

[Hoonz]

i have 2 highlifts and they don't bend lifting a GQ ute
ones an ARB one and the other is a black rat

[CAP51Z]

I bougth a cheaper one from Repco a few years back, used it once, on a zook, it bent & fell over. Wasnt on it long. Returned it, got a refund, then bought another (different brand) for an extra $20 from GRP, when it was in Auburn, has lasted & had no issues. Been used on SWB & LWB zooks, also my bros S2 LWB landie. Neither were actual HiLift brand.

[Maxtd5def]

Max,

That is normal, all hi-lifts do it.

A 1st year mech eng student could have told the original makers that it would happen (Eurler's column theory) and how it should be changed.

Funnily all of the immitators make minor changes, but they all build in the same shortcoming that gives them such a bad (and deserved) reputation.

They're nasty, but when you need em, you need em.

The hilift web site has a pdf file with some specs. The 7000 lb load rating is of course the shear pin. The 5' jacks have a rating of 2660 lbs in the top 12", but tested to 4000 lbs.

Unfortunately the 4' jacks we all commonly use have "N/A " listed.

The Maasdam site used to have some good specs about their Tiger Jack, but thats all gone.

Has anyone got any more load info?

TIA
Max P

p.s. - its a change to be able to research lb, ft & inches again, none of this metric crap!

[Tiny]

They're nasty, but when you need em, you need em.

True, but HiLifts and steel winch rope are the two things that make me uneasy

[just cruizin']

[TIA
Max P

p.s. - its a change to be able to research lb, ft & inches again, none of this metric crap!

Because imperial is SOOOO much easier right. 12 of this, three of that, 14 of some things else.:finger:

[HJ60_HEATHUS]

I released the hilift a bit to fast once, and the Cookie of a handle smacked me in the face. bloody hurt too :(

[pridhac]

This very nearly happened to me last week. Blimey, was I glad it missed.

I released the hilift a bit to fast once, and the ***** of a handle smacked me in the face. bloody hurt too :(

[Bush65]

so John how hard would it be to make a better one?
serg

It is not practical to modify an existing hi lift in any practical/useful way.

The main problem is that the mast is too slender in the sideways direction.

The load that can be supported is proportional to the inverse of the slenderness ratio squared.

Slenderness ratio is L/r where L is the effective height and r is the radius of gyration.

The effective height is height multiplied by a constant which accounts for how easy the ends can rotate. The easier that the ends can rotate, the less load that can be carried. With a hi lift jack, the base, how the mast is attached to the base and the condition of the ground affect this.

To see how this works - get something similar to a thin plastic school rule. Stand the rule on it's end and press down on the top to observe how it buckles. Now fix the bottom in a vice so that it is not free to rotate. It will take more load before it will buckle. If the top is constrained against rotation and not allowed to move to either side, it will carry more load again.

Radius of gyration is more difficult to explain. It depends on how far the material is distributed from the centre. For the same cross section area a thin wall circular tube has a larger radius of gyration than a solid bar.

This is where the greatest gain is possible over the standard hi lift. A rectangular hollow section mast can not buckle like the I section mast used by all hi lifts that I have seen.

Eccentric loading also reduces the buckling load capacity. The load is always going to be eccentric in the strong direction of a hi lift, but often it is eccentric in the weak direction because of how we place the jack in relation to the load and the condition of the ground that supports the base. The eccentricity is drastically increased if the mast is at an angle to the vertical.

[brighty]

so John how hard would it be to make a better one?
serg

It is not practical to modify an existing hi lift in any practical/useful way.

The main problem is that the mast is too slender in the sideways direction.

The load that can be supported is proportional to the inverse of the slenderness ratio squared.

Slenderness ratio is L/r where L is the effective height and r is the radius of gyration.

The effective height is height multiplied by a constant which accounts for how easy the ends can rotate. The easier that the ends can rotate, the less load that can be carried. With a hi lift jack, the base, how the mast is attached to the base and the condition of the ground affect this.

To see how this works - get something similar to a thin plastic school rule. Stand the rule on it's end and press down on the top to observe how it buckles. Now fix the bottom in a vice so that it is not free to rotate. It will take more load before it will buckle. If the top is constrained against rotation and not allowed to move to either side, it will carry more load again.

Radius of gyration is more difficult to explain. It depends on how far the material is distributed from the centre. For the same cross section area a thin wall circular tube has a larger radius of gyration than a solid bar.

This is where the greatest gain is possible over the standard hi lift. A rectangular hollow section mast can not buckle like the I section mast used by all hi lifts that I have seen.

Eccentric loading also reduces the buckling load capacity. The load is always going to be eccentric in the strong direction of a hi lift, but often it is eccentric in the weak direction because of how we place the jack in relation to the load and the condition of the ground that supports the base. The eccentricity is drastically increased if the mast is at an angle to the vertical.

Soooooooooo John.......... whens the first prototype you are going to build go on demo??? Surely if there is something that can be better built, stronger and not too much bulkier, there is a definite market there for it. Just proving it to the already "Hi-lift aware" public would be the hardest part wouldn't it??? But thats where the marketing comes in... a comparo side by side should prove it's worth against an "unlabelled" Hi-lift???

[uninformed]

John, at a guess what size would you have to make the rectangle hollow tube to meet the same load carring cap as a std hi-lift.

i'm guessing the hi-lifts just "climb" up the holes in the "I" section, so just drill some holes in the rhs and make new mech to fit???

serg

[Bush65]

John, at a guess what size would you have to make the rectangle hollow tube to meet the same load carring cap as a std hi-lift.

i'm guessing the hi-lifts just "climb" up the holes in the "I" section, so just drill some holes in the rhs and make new mech to fit???

serg

Have just done some quick calcs to get a feel for it.

If all else is equal, the buckling load capacity of slender columns is proportional to the cross section area multiplied by radius of gyration squared.

For a hi lift this is 14949 mm^3 (holes not included).

50 x 20 x 3 RHS -> 21296 mm^3 (142% capacity of hi lift)

50 x 25 x 3 RHS -> 36807 mm^3 (246% capacity of hi lift)

65 x 35 x 4 RHS -> 123740 mm^3 (800% capacity of hi lift)

Weight of mast is proportional to cross section area

hi lift -> 641 mm^2 (holes not included)

50 x 20 x 3 RHS -> 362 mm^2 (57% of hi lift)

50 x 25 x 3 RHS -> 392 mm^2 (61% of hi lift)

65 x 35 x 4 RHS -> 683 mm^2 (107% of hi lift)

[J Top]

Chrysler new all about these short comings when they built their bumper jacks in the 70's
J Top

[Maxtd5def]

G'day John,

So back to the original issue: is mine safe to re-use, or has the load rating been reduced? Is the rack elastic enough to bend safely a number of times, or is deformation occuring every time? At what point do you throw it away?

Regards
Max P

[Bush65]

G'day John,

So back to the original issue: is mine safe to re-use, or has the load rating been reduced? Is the rack elastic enough to bend safely a number of times, or is deformation occuring every time? At what point do you throw it away?

Regards
Max P

You said that it returned to straight when the load was removed. So it should be as good as before.

[Loanrangie]

I released the hilift a bit to fast once, and the ***** of a handle smacked me in the face. bloody hurt too :(

I had the handle of my hi lift fall on my head, fark it hurt and i saw stars for a while and had to ly down