That is a good catch RGB.
[quote]diesel25 wrote:
Dumbells are commonly constructed of Cast Iron.
Dumbells are most likely to fail in SHEAR. Cast Iron’s Shear Ultimate Strength is 240 MPa - or 35,000 psi.
But let’s add a BIG factor of safety to account for potential corrosion and fatigue: 0.5. This’ll make the shear Strength 17,500 psi.
We’ll take the dumbell handle width to be 1 inch ~ 25 mm
After ALL the factors of safety and VERY conservative estimations, your dumbell will not fail before pi^2*(25/1000)/4 x 17500 = 1,080 lbs
Its strength is likely significantly higher than that.
Load on dumbell when you drop it:
Newton’s 2nd law: F = m*a = (W/g)*a, where a = accleration as you push the dumbell (10 ft/s - you wish), g = gravitational acceleration (32.2), W = 500 lbs/dumbell (yeah right).
Then, F = (500/32.2)*10 = 155 lbs.
The weight of the dumbel is spread into two disks at the ends, each roughly half its nominal weight. Each end of the handle will be subjected to HALF the value of F in shear - 155/2 = 77.5 lbs
Factor of safety involved in the exercise = 1080/77.5 = 14.
Using 500 lb dumbells and pushing them up at 10 ft/s - both HIGHLY questionable) you would be 14x under the dumbells capacity.
This is AFTER our factors of safety accounting for potential corrosion and metal fatigue.
So, WHAT did you break again?[/quote]
You can cram your physics up your ass. Yeah, dbs are commonly made of cast iron - hex dbs. Many, many gyms have dbs that are NOT one-piece, rather, like others have pointed out, they are comprised of several plates that are either welded together or held together by an end-cap and a hex screw. These are the kind that WILL loosen and break with repetitive dropping. Get out a little more before you start telling us we’re full of shit.
DB
I’ve been in probably 30 different gyms while traveling and 90%+ of them have signs saying dropping weights is a cause for termination of membership (even in very hardcore gyms). The owner doesn’t want to replace them because they do break and there’s the possibility of injury. I’ve even seen a couple of signs saying “if you have to drop the weights, you’re using too much weight, lighten up”.
[quote]diesel25 wrote:
Dumbells are commonly constructed of Cast Iron.
Dumbells are most likely to fail in SHEAR. Cast Iron’s Shear Ultimate Strength is 240 MPa - or 35,000 psi.
But let’s add a BIG factor of safety to account for potential corrosion and fatigue: 0.5. This’ll make the shear Strength 17,500 psi.
We’ll take the dumbell handle width to be 1 inch ~ 25 mm
After ALL the factors of safety and VERY conservative estimations, your dumbell will not fail before pi^2*(25/1000)/4 x 17500 = 1,080 lbs
Its strength is likely significantly higher than that.
Load on dumbell when you drop it:
Newton’s 2nd law: F = m*a = (W/g)*a, where a = accleration as you push the dumbell (10 ft/s - you wish), g = gravitational acceleration (32.2), W = 500 lbs/dumbell (yeah right).
Then, F = (500/32.2)*10 = 155 lbs.
The weight of the dumbel is spread into two disks at the ends, each roughly half its nominal weight. Each end of the handle will be subjected to HALF the value of F in shear - 155/2 = 77.5 lbs
Factor of safety involved in the exercise = 1080/77.5 = 14.
Using 500 lb dumbells and pushing them up at 10 ft/s - both HIGHLY questionable) you would be 14x under the dumbells capacity.
This is AFTER our factors of safety accounting for potential corrosion and metal fatigue.
So, WHAT did you break again?[/quote]
I’m not going to go through this and find where you screwed your numbers up, but common sense and experience should be screaming at you that you messed up somewhere.
If you drop dumbbells, they have a tendency to loosen and later break. Even the heavy, single-piece, cast-iron Hex dumbbells will bow in the middle if they’re dropped. It does not take 1,080 lbs to break a dumbbell.
There are ton’s of different ways dumbells can be made. i have seen the hex one piece and the put together handle with plates either wleded or bolted on. If you have seen many 140LB hex one piece dumbells you must be lifting at a great gym. Most places I lifted only have the one piece hex DB’s up to a certain wieght then the rest are the build a DB. I haev seen an 85LBer which cracked at my old gym, one side ended up with a crack in one of the 10LB plates and a nice sized chunk broke off while a buddy was lifting. If the thing had broken all the way through instead of losing a piece it could have been much worse, but he was only doing row’s it wasn;'t over head anyway. anything can and will break if given the right circumstances so why not limit the chances.
however I do on occasion drop them under control if I am lifting without a spot on DB press, but never any other exercise. the whole putting on your knee thing gets more and more difficult once you are working triples and 5X5 and hit the last set or two.
BB
Damn, you mean the dumbbells I have seen broken from being dropped, do not really exist? Clearly the I.E.O.R. (Internet Engineer on Record) has shown that I was hallucinating… guess I need to lay off the LSD before lifting.
[quote]dollarbill44 wrote:
You can cram your physics up your ass.
DB[/quote]
Should I start by cranking it out of yours?
[quote]rgb321 wrote:
diesel25 wrote:
Dumbells are commonly constructed of Cast Iron.
Dumbells are most likely to fail in SHEAR. Cast Iron’s Shear Ultimate Strength is 240 MPa - or 35,000 psi.
But let’s add a BIG factor of safety to account for potential corrosion and fatigue: 0.5. This’ll make the shear Strength 17,500 psi.
We’ll take the dumbell handle width to be 1 inch ~ 25 mm
After ALL the factors of safety and VERY conservative estimations, your dumbell will not fail before pi^2*(25/1000)/4 x 17500 = 1,080 lbs
Its strength is likely significantly higher than that.
Load on dumbell when you drop it:
Newton’s 2nd law: F = m*a = (W/g)*a, where a = accleration as you push the dumbell (10 ft/s - you wish), g = gravitational acceleration (32.2), W = 500 lbs/dumbell (yeah right).
Then, F = (500/32.2)*10 = 155 lbs.
The weight of the dumbel is spread into two disks at the ends, each roughly half its nominal weight. Each end of the handle will be subjected to HALF the value of F in shear - 155/2 = 77.5 lbs
Factor of safety involved in the exercise = 1080/77.5 = 14.
Using 500 lb dumbells and pushing them up at 10 ft/s - both HIGHLY questionable) you would be 14x under the dumbells capacity.
This is AFTER our factors of safety accounting for potential corrosion and metal fatigue.
So, WHAT did you break again?
The stress is a combination of shear and moment
A 500 LB dumbell with a 1" bar would see a max static stress of about 15,000 psi
When dropped from a height of 3 feet the impact stress would be multplied by 3.6, from 6 ft multiply by 4.7
repetetive impacts could cause crack propogation and eventual failure on larger dumbells, especialy the 3 piece dumbells where the weights are fastened to the bar with a bolt.[/quote]
Owned.
[quote]diesel25 wrote:
dollarbill44 wrote:
You can cram your physics up your ass.
DB
Should I start by cranking it out of yours?[/quote]
I think your protective helmet is on too tight, Einstein.
DB
[quote]dollarbill44 wrote:
I think your protective helmet is on too tight, Einstein.
DB[/quote]
Not used to wearing one for breaking dumbells.
[quote]diesel25 wrote:
dollarbill44 wrote:
I think your protective helmet is on too tight, Einstein.
DB
Not used to wearing one for breaking dumbells.
[/quote]
Who you callin a dumbell, dumbell???