Who's got their engineering cap on tonight?

[1 Wild R/T]

I'm building a hoisting rig to transfer a load inside a enclosed space, I have limited room for the rigging & I'm trying to figure how the load offset effects things...

Basically I need to hang a hoist off a pair of 20mm solid hardened steel shafts, these shafts will be cantilevered out, the cable will be deflected around a sheave 90 degrees back toward the anchor point, it will then travel 14" back under the anchor point & do another 90 degree bend down to the load... The frame supporting the two sheaves will also be supported by those two 20mm rods...

I know the rods will deflect, question is how much & that depends on where the majority of the force is centered... Is it where the rig attaches to the rods or is it where the load actually is hanging? Or somewhere in between?

 

[Kern Dog]

Without going into too much brain work....
In construction, when we cantilever a ceiling joist or a rafter out past a bearing point, for every foot that is unsupported, the wood has to be inboard by 2 feet. If you have a 2 foot overhang, we have to have 4 feet inside of the point of bearing.
This is probably worthless to you but **** it...I miss framing and I refuse to wear a mask.

 

[66 Sat]

Can you post up a simple sketch of what you have?

 

[1 Wild R/T]

Okay I guess I'll explain further.... I have a neighbor who's a quadriplegic... In the past to transfer him from his wheelchair to his vehicle he's had a super sketchy frame of 2x4's that a friend of his built... It was junk when new & only got worse... He got dropped a couple times...

Anyway he recently got a newer vehicle (an SUV) & his buddy was gonna redo the wood hoisting frame... I got myself involved & I'm using some industrial spare parts that I plan to rig off the roof rack... I have four linear bearings & two shafts.. I've sourced some cable sheaves & I've machined up a few parts so far... I plan to use a small electric winch

I'm attaching the linear bearing to the roof rack, the shafts will slide out to raise him from his chair then slide in to transfer to the passenger seat in the SUV...

If I didn't already have the 20MM shafts I'd probably over do it & use 1.25" shafts. & bearings but he's on a fixed income so I'm doing this out of my pocket, I have parts, I plan to use them... I know they will deflect, just don't know if it'll be 3/4" or 2" which is where knowing how this load will bear matters..

 

[Photon440]

There are too many variables. Knowing the length of the 20 mm. bars would help, and their composition. Merely being 'hardened' isn't enough. Are they case hardened or oil quenched? What is the total load, including person, chair, bearings and winch hardware supported by the bar?

You may have to do some experimentation on your own; using a single shaft, place a 100 lb. load on the end and measure the deflection. Try again with 150 lb. and you'll see what a 300 pound load would do to two bars.

Also, the 'L' shape between the winch and the load, is that a solid piece that would have it's own deflection to add in?

 

[1 Wild R/T]

20mm bars are 5' long, max extension will be 30" so 50% of the bar.. The bearings will be as close to 30" apart as I can get them... Chair has no bearing, he'll be lifted from the chair & transferred to the seat in the SUV..

The rods are Thompson shafting which is induction hardened to a depth of approx 4mm

I've dug up the OE spec for deflection & at 24" it should be close to 3/4" but at 36" it's 1.65" so I'd like to know if the fact the actual load is 14" back from the end of the rod do I treat it as 30" or something less.. Seems like I recall seeing a demonstration that tested this sort of load & wasn't at the end of the bar...

The winch is 14lb, the assorted brackets & such I don't have a number for yet but I'm trying to keep the weight under control without sacrificing strength..

My neighbor weighs #165-170...

 

[66 Sat]

Mate, you're doing a good job for someone, that's the main thing.

Now I'm not an engineer...but I think that if the cable is supported by an eyelet or pulley under the load, then the force will be shared by the cantilevered beam to some degree, and the bending moment won't be between the winch and the last bearing. How much, I wouldn't have a clue, but I believe you're on the right track with your thinking.

 

[moparedtn]

Without going into too much brain work....
In construction, when we cantilever a ceiling joist or a rafter out past a bearing point, for every foot that is unsupported, the wood has to be inboard by 2 feet. If you have a 2 foot overhang, we have to have 4 feet inside of the point of bearing.
This is probably worthless to you but **** it...I miss framing and I refuse to wear a mask.

 

[Richard Cranium]

I got my engineer cap....

 

[Roger63]

Have you looked at the mobile lifts?

 

[Mike67]

My concern would be the roof racks points of attachment, the furthest point having the to bear the upward force created at the fulcrum ( would be the drivers side roof rail).
Essentially what you are creating is a lever if I understand your drawing, the longer the lever the greater the force at the opposite end.

 

[hanks73340]

Just don't end up like this.....

 

[1 Wild R/T]

Have you looked at the mobile lifts?View attachment 1081903

Great, we can get him into the car... How do we get him out at his destination? Trust me I know I can buy a solution but since he has limited funds & his insurance won't cover it.... Have you priced any medical type equipment? They certainly charge a premium...

Heck, the easy way would be a handicap van... But again $$$$$$$$

 

[1 Wild R/T]

My concern would be the roof racks points of attachment, the furthest point having the to bear the upward force created at the fulcrum ( would be the drivers side roof rail).
Essentially what you are creating is a lever if I understand your drawing, the longer the lever the greater the force at the opposite end.

I agree it is a concern... Especially when I initially found the recommended dynamic load for the roof rack.. But after talking with Thule the low dynamic is because when driving down the road wind & G forces apply... I have a static load & the numbers are much higher... Should be fine... I'll be testing, unlike the previous mess I don't plan to drop him...

 

[Roger63]

Great, we can get him into the car... How do we get him out at his destination? Trust me I know I can buy a solution but since he has limited funds & his insurance won't cover it.... Have you priced any medical type equipment? They certainly charge a premium...

Heck, the easy way would be a handicap van... But again $$$$$$$$

Just trying to evaluate the situation so I could possibly help. Yes I have priced the equipment I spent 6 months paralyzed.

 

[1 Wild R/T]

Just don't end up like this.....
View attachment 1081919

He already had that.. LOL.. I hope I can do a little better..

 

[Mike67]

Here is the equation related to your drawing.
I understand what Thule is saying , but I think their estimates are based on the static load being dispersed across 4 points of attachment at the roof. Even when wind shears are applied to the rack the "lever effect" is too short to make a difference. I think your configuration could.... as I only see the drivers side rail ( fulcrum), passenger side rail having the upward force applied by the load at the winch.
DISCLAIMER: I am not an engineer by any means, but in my previous life as a commercial electrician I had to come up with a lot of lifting and cable pulling methods that often included weights of 10k lbs.or pulling capacity..
Im not trying to squash your idea and admire your willingness to help but would hate to hear of someone injured.

Hope this helps!

 

[1 Wild R/T]

Just trying to evaluate the situation so I could possibly help. Yes I have priced the equipment I spent 6 months paralyzed.

Trust me I appreciate the help... This guy unfortunately falls through to many programs, when my roommate got injured his insurance covered more than anyone would believe (he was college staff, they have great insurance) but my neighbor isn't so lucky.... And since he's been a quad for 35+ years what money he had is long gone... So it falls to friends & neighbors....

 

[Roger63]

Trust me I appreciate the help... This guy unfortunately falls through to many programs, when my roommate got injured his insurance covered more than anyone would believe (he was college staff, they have great insurance) but my neighbor isn't so lucky.... And since he's been a quad for 35+ years what money he had is long gone... So it falls to friends & neighbors....

I understand sir I had no insurance also.

 

[Mike67]

Have you thought of maybe making something like a seat that turns sideways and maybe slides out and drops down ?
Again you have my utmost respect trying to help!!!!