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MG MGA - Lifting Advice Needed
|My schoolboy applied maths knowledge is fast disappearing, so structural engineering advice needed.|
Many of you will know that I have a mobile car ramp/hoist. When not in use I want to store it on its side against the garage wall. There is a video of this being done and is relatively easy for 2 strong people. As I am, more often than not, on my own I am looking at using a block and tackle.
The hoist weighs 260 kg and is 4 feet wide. The top anchor point for the block and tackle will be 3/4 along an 18 foot roofing joist made of 3 x 2 timber. It is 8.5 feet above the floor. The joist is part of the classic construction, forming a triangle with reinforcing struts and hangers. I have fitted a strong bracket to the wall under the joist centre point and also one at the proposed lifting point.
My proposed hoisting arrangement is attached. Should I be ok?
|Steve, I don't know the answer to your question but I love your artwork!|
|Yes. You are only pulling half the weight, as one side of the list remains on the floor. The truss you attach to should have some lateral bracing on the bottom run so it will not bow sideways.|
Is your joist really 3 x 2? A couple of photos of your structure would be helpful in assessing your situation. I'm no engineer, but I have hoisted my MG engine and transmission from a double 2 x 12 joist (actual dimension 3" x 11 1/2") about 15 feet long, using a "come-along" cable hoist. I also have a wooden deck at my cabin that hinges up, somewhat like you propose, so I don't have to shovel snow from it. I use a boat trailer winch to hoist it up. The anchor is just screwed into a wall stud.
|k v morton|
It took 5 mins on PowerPoint. I was going to put a few more features: eyes, nose, etc!!
Yes, I was thinking about the lateral movement. I will give it a bit more thought. Might put a few more belt and braces brackets in. I was originally struggling with thoughts of the initial lift; was it 50%, 60% etc of the weight. I guess that is what started off my enquiry. I have ordered a 1-ton capacity block and tackle which should be adequate.
It is the initial lift that nags me. I suppose if I am unhappy with the strain on the joist I could use my mobile engine hoist to take the first 20 degrees off the lift.
I guess once I have done the first lift I will be happy.
|"I was originally struggling with thoughts of the initial lift; was it 50%, 60% etc of the weight."|
Your block and tackle will lift the hoist without any problem. My concern would be for your upper attachment, and the bracing of the lower chord of the truss as Barney has indicated. That would be the point of the major stress.
|George Raham [TD4224]|
|The total force on your upper attachment point is what you must consider. this will be made up of two forces, ! the load of lifting the the hoist and 2. the force you are pulling with on the rope.|
1. The force needed to begin the lift of the hoist will be more than half its weight because you are pulling on an angle not straight up. Of course this force will lesson as the hoist is lifted, but is the max force that you must plan for.
2. The mechanical advantage with your block and tackle should be one half, so the pull on the rope would be 1/4 the total weight of the hoist plus the friction in the pulleys.
Total force to just begin to lift it off the ground should be more than 3/4 the weight of the hoist.
of course this is all predicated on my memory of 9th grade physical science.
|S E Bryan|
|I must amend my statement. The total force will be close to or more than the weight of the hoist. the 3/4 figure is for a straight up pull, and yours is at an angle. |
Remember too that the inertia of the hoist as you begin to lift it and slow its decent to the floor when lowering must be added on too.
|S E Bryan|
|Steve, is this the area that you are mounting the hoist? Is it on the truss closest to the wall? As long as the lower chord is securely fixed to the wall at a couple of points and there are no splices in the section, I don't see any problem. Why not just mount the hoist to the wall?|
The photo was still kicking around my desktop from your previous construction inquirery.
|When I say why not wall mount the hoist, I mean the block and tackle mechanism not the car lift. How thick is the wall?|
|On the basis of what you advise in the great pic and the description the beam is NOT up to it . A 3 by 2 beam 18ft long has zilch strength vertically and even less horizontally. I presume the beam is not fixed to the wall or braced or fixed horizontally at or near to the point of lift. |
My considerations ( since this is not a one off operation but a routine)..
.....The minimum dynamic shock load is a factor of 2 above static load ..so if it jerks even slightly whilst being lifted..yaroo watch your toes!!!.
...The angle to the vertical at lift off is about 27deg ( allowing for an extra 6in from the wall) which gives a horizontal component of load of 45% of the weight being lifted. So based on following para. estimate the horizontal load for design should be 230kg.
....I would allow for the total load of 260kg ( since this is not a one off temp. operation ) and a dynamic safety factor of 2 i.e. 520kg. Safety factors are important for routine ops..they can be cut down for one off temps... and you only have one set of toes.
I would suggest the following...
...a vertical prop under the point of lift of same beam dimensions or brackets under the beam at the point of lift . You mention this but make sure they are heavy duty . If the bracketts enclose the beam ( i.e. have horizontal strength then they are of use re. the next para..
...a horizontal prop between the point of lift and a hard point..or fixing it securely with 10mm or greater bolts and load spread plates (say 4by 4in behind the wall ) )to the wall close to the lifting point and at two other points about 50 cm away from the lifting point to spread the lateral load over the wall . Single brick Walls are very weak in horizontal direction.
Block and tackle strength looks fine.
Why not use the engine hoist for the whole operation???
|Okay, here is the science behind it. This is not 18-foot span by a single board. Take a look at his picture. This is a triangulated roof truss with angle struts in the shape of "W" and vertical braces at the apexes. On 24-inch centers this should support roof loafing of 40 to 50 pounds per square foot plus ceiling weight and/or attic floor load of at least 20 pounds per square foot. One truss has a design load rating well in excess of 2000 pounds (distributed over the full length). You can walk on the bottom span anywhere along the length as though it was the attic floor, so there is no problem applying this intended (relatively light) lifting load anywhere on the span between braces.|
I have very similar roof trusses in my garage with 22-foot span, and I lift an engine with a cable jack on a fat lag-screw eye-bolt (and have been doing it for more than 30 years). The trick is to install the lag screw at one of the lower apexes of the truss where there will be zero span loading, as all of the truss parts are in tension or compression load.
Triangulated roof trusses are designed to support huge vertical loads with minimal side load. If you would apply any significant lateral load on the truss you need to run a transverse brace across multiple trusses, preferable all the way to the end walls. If there is a ceiling of drywall installed, that will serve as the lateral bracing spanning multiple trusses.
|Steve, can you confirm Barney's assertion that the beam is the bottom member of a roof truss? This is fine except roof trusses in this country are more usually out of 3x1 1/4" or 3x1 1/2". Haven't seen any 2" wide trusses but no doubt its possible.|
|J H Cole|
|Hi Steve - interesting physics moments problem and love your diagram . Not sure if you are asking what the force would be on the wall support or the force you would need to pull on the block and tackle.|
From the initial pulling position when the hoist is horizontal - assuming the rope has say a 45 angle - the initial pulling force in the end and wall support will be reduced to around 183kg. (using moments 260kg x 2 = F cos45 x 4) This will of course increase gradually as the hoist is raised to the vertical ( when it will be 260kg straight down from the wall)
|Cam, Interesting info but I think your formula might be a bit off. I'm sure the pulling force becomes less as you lift. Eventually, at the vertical position, the lifting load becomes zero as all the weight transfers to the resting edge of the lift. At that point, Steve will have to secure the top of the lift against the wall to prevent it from tipping.|
|When I saw the picture of the roof in John's photo, I thought, yes, that looks very similar to mine. Then I realised it was mine! He must have got it from the archives from my previous enquiry about boarding in the loft space and cutting a joist to provide greater access - I did board it in, but I did not cut a joist.|
I have just replaced the supporting bracket on the central buttress in the photo with a heavier duty version and also screwed it up into the joist - 10% discount day for us oldies in B&Q.
Having thought about this overnight, I am going to see where I get to using the mobile engine hoist, with the block and tackle (just arrived) taking up the slack. I will time the operation and take some photos. Might be tomorrow rather than this afternoon.
Cam, I can't agree with you about the last statement saying that the load will gradually increase. I am not lifting it off the floor. In fact, by the time it gets to about 60 degrees I will probably be able to push it the rest of the way with only a little assistance from the lifting gear.
|Steve, the max force needed to lift the ramp is when its on the floor. This requires 144kg (onto truss) but 72kg on the 2 block pulley rope. The force gets less as it rises so at say 45 degrees the manual force to continue rotating the ramp would be 98kg which as you say is just about manageable by a fit person.|
I would load test the truss with about 130kg (approx max vertical load) and sight along the bottom for deflection. Check that its no more than about 3/4" and I doubt whether it will be. It would be worth strutting the bottom member against the next truss.
|J H Cole|
|All done. Very successful and quite easy thanks to all your inputs. certainly all the comments made me feel quite relaxed when taking the loads.|
I used a combination of my engine hoist and block and tackle. I led with the engine hoist, using the block and tackle to steady and pull the top inwards, rather than lift.
I started at 1400 hrs by unpacking the block and tackle and closed the garage door at 1429 hrs. Much quicker and easier than I expected
Hope i can put this over so you can understand, As there is no way i could post a drawing i can only just find the switch that turns this PC on :-)
In your diagram the block is slung on the roof joist and them fastened onto the ramp. This makes the forces on the joist pulling on the diagonal.
Now if you could fabricate a roller bracket up to be mounted at a point (to be worked out) lower down the wall, Then the lifting rope/chain would fit between the wall & roller bracket. The force has now been moved from the joist to the wall, Which is easier to brace up
The force on the block would be straight down ( Therefor no side stresses) As for the wall im unsure if its brick/stone or wood. If its wood it may not take the stresses. If its Brick/Stone then Bolts/Screwed rod going all the way through with spreader plates both sides would i think take the load.
I only offer this up as it seems the opinion is will the joists take the side way stresses.
Hope that makes sense.
I know what i mean:-)
|P D HEWITT|
|You are right Chuck and Steve - for some reason I imagined it lifting off the floor.|
|Steve....great ( and safe) solution...|
|Steve..how did you find using the hoist? I am particularly interested in access to the centre of the underside as I looked at similar devices and was concerned about this aspect.....|
Although I have tested the lift I have net yet lifted the car. It has been just too cold for me to do any productive work on the car this last month. That all said, I am confident it will do the job for me.
The main issue with this type of hoist is its width. There is only 2 inches to spare when I push it under the car from the front or back (it can be used sideways). It is therefore somewhat in the way when I want regular car use and is the reason why I have now stored it on its side.
I won't get a chance to use it until the end of next month as I am coming over your way next week for a month - seeing Barry Gannon etc. Once I do get the hoist in action I will report back with my views.
There is a German video on Youtube showing this hoist in action. It gives you some indication of access to the underside: http://www.youtube.com/watch?v=M_IbLy2ixAo&noredirect=1
|just watched the video and access was not too bad......good bit of german engineering ...|
|It looks a neat storage solution Steve - excellent.|
This thread was discussed between 14/02/2012 and 16/02/2012
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