This seems rather clear intuitively, but my intuition has been known to fail me: If I am using 4 casters for a 400lb gross weight table w/ machinery, will 4 100lb casters be right (putting aside issue of location of each caster for the moment)? Or, is there a rule of thumb that each caster should have X% capacity above its share of the load?
I am considering some simple Harbor Freight casters. The table I need to move will not be moved regularly -- just when I need to rip something 8' long.
The table is already built (recycled from another use). I am trying to keep the casters as small in diam as possible to keep the overall height down.
I don't have a clue on how to rate these things. Especially if the casters are chiwaneese. Having said that, I made a base for my JWBS-20 using casters rated at 125# each which is about what it weighs.
Then I started thinking about what would happen if one failed since having a corner drop > 4 inches across a 2' x 3' rectangle with a 6' tall piece of machinery attached. In my case, I think it would tip over and I would be sick for months.
So, I screwed two boards to frame that are just a bit shorter than the height of the wheels. So if a caster breaks, the saw would only drop
1/2" I'm not good at ascii art so I hope you see this in your mind. By cutting a ramp on opposite ends of each board, I can tap in a wedge to secure the saw in place.
Well (as usual) the ASCII did not help ... but the text did. Makes sense. Actually, I had been thinking of something similar with bolt-adjustable feet so I can lift the wheels up -- since I will rarely move the thing (I think) and this way I can skip cost of lockable wheels. Your wedge idea sounds simpler, yet effective. Indeed, rather clever. Thanks.
Just to be clear, you used 4 125lb casters for a unit that weighs _in total_ 125lb? Does it roll well? Are they cheap casters?
There are a lot of unkowns here. First of all, is the manufacturer's load limit an absolute maximum or does he include a safety margin? Are some manufacturer's pushing the limit for advertising purposes (low cost but high load limit?).
Another important questions are whether the load is evenly distributed over the casters or not. A 400 lb machine will put a 100 lb load on each caster only if the weight is evenly distributed. Also, what is the nature of the floor? Is it smooth and regular of does it have lots of bumps and such? In your case, we can assume that there are no steep inclines, which otherwise would be a factor.
Also, if there is a failure, generally, only one caster will fail and if it is, for example, a 4" caster, that does not mean that the table will fall 4". The wheel, bearing or axle pin (not the caster mounting bracket) are most likely to fail and the maximum drop would be the distance from the floor to the bottom of the mounting bracket. If the load was evenly distributed and the table was not moving at a high rate of speed over a large bump (or dip), the table would probably sag but would be unlikely to be in danger of falling over.
So, in general, I think for intermittent use in a home workshop under most conditions, you could use casters rated for the maximum dead weight load (table plus equipment) divided by four.
When a reputable manufacturer rates a caster at, for example, 200 lb or use on a concrete floor, the rating is for an evently distributed load moving at a normal walking or towing speed (no more than 3-5 miles an hour) and occasionally passing over expansion joints and threshold plates.
Something to consider is that no floor is perfectly flat and no cart has four wheels perfectly in one plane. Therefore the load will be shared by three wheels a good share of the time. As the cart is rocked imperceptibly from side to side, the load momentarily is shared by two wheels.
Also the center of gravity of your load won't be perfectly in the center of the wheel pattern. At the most, load the wheels to 1/2 their nominal rating. Less if you are going to move it much.
You take the total capacity of all 4 casters and compare it to the total load. Of course, you have to allow for any imbalance that exists or might occur.
Without knowing if the 100lb. capacity is a true 100lbs., your best bet is to build in a wide safety margin, say a minimum of 50%. I'd even be tempted to double the capacity versus the load.
4 x 125 unit weighs ~ 500 lbs. Rolling is not as nice as tool box casters with ball bearing axles. As far as cheap, 2.99 each so yup, cheap. Biggest issue with rolling is that machine is top heavy so anything like a nail can be a problem as a potential tip over. Tonight I ran into a 16p nail on floor and the saw stopped immediately with a slight tipping action. Not a problem since I expect issues with moving a bandsaw on a mobile base and go slow and hold on tight.
Capacity is not a big problem, wheel diameter and construction is more important, IMO. Any four quality casters can handle the load. The problem with small casters is they do not work well unless the floor is near perfect. A wood chip can stop them. Just like going over a curb, a wagon will have a problem, a truck tire much less so, but a 4' diameter tire has no problem at all.
I have a table with 2" casters. They will be replaced by 4" soon for the reasons I mentioned. Ed
I am also dealing with caster issues, and have spent time on the web emailing customer support desks and talking with local caster companies. Truth is, the type of caster is very important, which depends on the floor type, the temperatures, presence of grease/acid, etc., how fast the equipment will be moved, and whether it will be moved gradually or quickly. My suggestion is to talk with the experts - companies that make casters for industrial equipment. In my case, they recommended 4 casters with 250lb rating, made of phenolic plastic. It is overkill in terms of weight rating, but at $4.87 per caster, I have piece of mind.
I can see it now: My table with 4' wheels -- or maybe just 30". Kind of like a wheelchair setup. Or some old vendor carts. Yes, I can see how that would be smooth. Gee, I wonder of there is a cheap source for beat-up wheelchairs I could recycle?
Well, in my current situation, we are talking about moving the table a few feet -- my shop is only 13x20 or so. More like that old pocket-sized game where you move the tiles around w/in a frame to get them in order. I _will_ have to look out for those wood chips! Many of them are suicidal, jumping out just as they see you coming.
Yeah...!!!! Well actually the larger Hard rubber wheels worked out OK but you run into the issue of raising the tool too high. My old TS sat a good
3" taller than my present Cabinet saw on its mobile base..
You might consider looking into replacement wheels direct from HTC. Those small diameter wheels and casters are small and hold up to the weight real well and roll over irregularities in my concrete floor with little feed back.
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