I have arrived at Compromise Crossroads and I am not happy about it.
The offender is the jack assembly design for the cart.
It seems I can have it:
• serviceable
• strong
• simple
(pick any two)
...and this choice offends my delicate sensibilities. Clearly, 'Strong' must be one of the choices otherwise why bother?
Normally, my second choice would be 'Serviceable', because serviceability has long been part of my philosophy, my raison d'être, or one of 'em at least, and in the past I've never shied away from over-designing things.
[noises off: "NO, REALLY?"]
Quiet in the peanut gallery.
A simple outrigger jack would consist of a long nut (AKA "coupling nut") welded to the end of the outrigger, a piece of threaded rod run through the nut, and a crank handle of thin bent rod welded to the end of the threaded rod. That's it! And that is where I started in my thinking, but the more I thunk on it, the less I liked it.
It has the advantage of simplicity. It is also fantastically weak. The distance from the bottom of the outriggers to the floor is 7.5in or so, making a long lever for any side loads on the cart, and threaded rod is soft and relatively low strength. Own the other hand, if we specify some ginormous threaded rod, then materials become expensive and it might still be weaker than a jack with a sliding ram (moved by a reasonable-sized threaded rod) which is supported by a close-fitting housing.
It is not horrifically difficult to make a sliding ram setup, although there is always the problem of obtaining sliding-fit tubing with sufficient wall thickness. But that's if it can't be disassembled.
The offender is the jack assembly design for the cart.
It seems I can have it:
• serviceable
• strong
• simple
(pick any two)
...and this choice offends my delicate sensibilities. Clearly, 'Strong' must be one of the choices otherwise why bother?
Normally, my second choice would be 'Serviceable', because serviceability has long been part of my philosophy, my raison d'être, or one of 'em at least, and in the past I've never shied away from over-designing things.
[noises off: "NO, REALLY?"]
Quiet in the peanut gallery.
A simple outrigger jack would consist of a long nut (AKA "coupling nut") welded to the end of the outrigger, a piece of threaded rod run through the nut, and a crank handle of thin bent rod welded to the end of the threaded rod. That's it! And that is where I started in my thinking, but the more I thunk on it, the less I liked it.
It has the advantage of simplicity. It is also fantastically weak. The distance from the bottom of the outriggers to the floor is 7.5in or so, making a long lever for any side loads on the cart, and threaded rod is soft and relatively low strength. Own the other hand, if we specify some ginormous threaded rod, then materials become expensive and it might still be weaker than a jack with a sliding ram (moved by a reasonable-sized threaded rod) which is supported by a close-fitting housing.
It is not horrifically difficult to make a sliding ram setup, although there is always the problem of obtaining sliding-fit tubing with sufficient wall thickness. But that's if it can't be disassembled.
 |
| (conceptual design, not to scale) |
One important factor that affects serviceability is usage; how many hours a day or year is the thing operated? (aha!)
In our case, the average usage over a year isn't zero, but it's really low. This makes me reconsider the evils of a sealed (welded) mechanism. The ram would still come out, giving access to the screw for lube. But the screw and its thrust bearing would be permanently trapped by the top bulkhead of the ram housing.
Oh, hey, what if the crank handle was removeable such that it didn't munge up the rod too much, so then the rod and its thrust bearing could be dropped out the bottom of the housing (after removing whatever retaining collar is on the top of the housing bulkhead)?
If the bearing (which might not be more than a plastic or brass washer) is just loose on the rod, then it's easy to replace (or grease).
This is do-able with a pin through the rod and crank, or a hole through the rod for the crank handle which is secured with a set screw in the end of the rod. Having it welded on was the only obstacle to complete serviceability. I have to reiterate: I really, really REALLY don't like assemblies which cannot be disassembled; makes me see red! (except things which aren't meant to ever come apart, obv)
"No User-Serviceable Parts Inside, Refer Servicing to your mom, shut up!" 😂 Yeah, I hate that, especially when there are fuses or other replaceable parts concealed inside. Cough tire pressure transmitters cough.
I haven't much in the way of sketches for this yet, tho I have the concepts and images mostly fixed in my head now, (helped by writing this down) so drawings would be more for you, dear reader, than for me... and it's going to be a while before I can buy ANY more steel, unfortunately.
The housing and the ram are square steel tubing having a sliding fit between the two. Most of the ram will remain in the housing when in use, for strength. The length and placement on the outriggers will be such that only 2in - 3in of extension will be needed to reach the floor. Hmm, how much ground clearance do I want to sacrifice? It has those lovely tall casters you see... but then that counterweight hangs down like a toxic udder... this may be less of a deal than I thought.
The active nut in the top of the ram will probably be nothing more than a coupling nut intended for joining threaded rods. Lots of thread length for strength, COTS and cheap. OTOH, if I fabbed the nuts from scratch out of brass, the crank would probably be easier to turn, but then I'd have to solder them into the rams? Meh. It'll be greased, obviously.
The "collar" shown around the rod on top of the housing is just a shaft collar with a set screw to keep the rod from falling out; it sees no forces. The "collar" on the rod inside the housing is the bottom surface of our thrust bearing, firmly secured in some fashion to the rod. I could get all fancy and make that a heat-shrink joint but remember I gotta make four of these things. Exact feature design TBD while not forgetting that the forces on a single jack screw could be 400 - 500lbs. If this is summed on a 1in brass washer with a half-inch hole, it sees 675PSI of pressure. This will not deform brass, but brass can still gall unless the bearing surfaces are polished. I'm not sure about bearing plastics, I'll have to look them up. Might perform better.
The heavy duty machine feet I want for the ends I already happened to have in my box of feet.
PS: sorry about the smudges on the sketch, I forgot you're supposed to have clean, dry hands. There is a lot unsaid in that drawing. Also, my masterful hatching (cough) made the rod left-handed which isn't right or necessary. :)
If the bearing (which might not be more than a plastic or brass washer) is just loose on the rod, then it's easy to replace (or grease).
This is do-able with a pin through the rod and crank, or a hole through the rod for the crank handle which is secured with a set screw in the end of the rod. Having it welded on was the only obstacle to complete serviceability. I have to reiterate: I really, really REALLY don't like assemblies which cannot be disassembled; makes me see red! (except things which aren't meant to ever come apart, obv)
"No User-Serviceable Parts Inside, Refer Servicing to your mom, shut up!" 😂 Yeah, I hate that, especially when there are fuses or other replaceable parts concealed inside. Cough tire pressure transmitters cough.
I haven't much in the way of sketches for this yet, tho I have the concepts and images mostly fixed in my head now, (helped by writing this down) so drawings would be more for you, dear reader, than for me... and it's going to be a while before I can buy ANY more steel, unfortunately.
The housing and the ram are square steel tubing having a sliding fit between the two. Most of the ram will remain in the housing when in use, for strength. The length and placement on the outriggers will be such that only 2in - 3in of extension will be needed to reach the floor. Hmm, how much ground clearance do I want to sacrifice? It has those lovely tall casters you see... but then that counterweight hangs down like a toxic udder... this may be less of a deal than I thought.
The active nut in the top of the ram will probably be nothing more than a coupling nut intended for joining threaded rods. Lots of thread length for strength, COTS and cheap. OTOH, if I fabbed the nuts from scratch out of brass, the crank would probably be easier to turn, but then I'd have to solder them into the rams? Meh. It'll be greased, obviously.
The "collar" shown around the rod on top of the housing is just a shaft collar with a set screw to keep the rod from falling out; it sees no forces. The "collar" on the rod inside the housing is the bottom surface of our thrust bearing, firmly secured in some fashion to the rod. I could get all fancy and make that a heat-shrink joint but remember I gotta make four of these things. Exact feature design TBD while not forgetting that the forces on a single jack screw could be 400 - 500lbs. If this is summed on a 1in brass washer with a half-inch hole, it sees 675PSI of pressure. This will not deform brass, but brass can still gall unless the bearing surfaces are polished. I'm not sure about bearing plastics, I'll have to look them up. Might perform better.
The heavy duty machine feet I want for the ends I already happened to have in my box of feet.
PS: sorry about the smudges on the sketch, I forgot you're supposed to have clean, dry hands. There is a lot unsaid in that drawing. Also, my masterful hatching (cough) made the rod left-handed which isn't right or necessary. :)
So now I can see how to make it both Serviceable and Strong, but 'Simple' has left the chat.
If I make additional sketches, I'll update this post later. I'm finding it much easier to continue making sketches with pencil and paper than with the new CAD program I'm (supposed to be) trying to learn (FreeCAD).
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