The Hoisting Cart project, part 2: The Mast

 The mast is a fairly critical part, in that it must support the weight of the load, but because there is a two foot boom between the mast and the hoist, it must also support a significant bending moment, which is applied sideways onto the mast by the jib housing.  For this reason, I decided to use a brand new piece of stock tubing whistled up from McMaster-Carr.  The tube is mild carbon steel, 3in. OD, .125 wall, and about 7ft long, cut down from an 8ft piece.  I literally had to save for a couple of years to afford it, but was determined not to worry about rust-introduced pits and weak points.

<-- The height of the existing light fixtures in the shop determined the height of the mast.

 The mast connections to the cart have already been described.  Where the holes for the retaining bolt (in the bottom bracket) were made, the mast was reinforced with a tube, like so:

This spreads any loads on the bolt over a much larger area than the thin walls of the tube, ensuring the holes in the tube do not deform over time.








 The load on the boom is supported by two bearings on the top of the mast; a thrust bearing to take the vertical load, and a pilot bearing to prevent the boom from simply sliding off of the thrust bearing.
 

Half of each bearing is connected to the top of the mast (the inner part in the drawing), the other halves are part of the jib housing (the outer part in the drawing).

I changed the design from what is shown in this sketch, instead using an inch thick piece of steel to hold the pin (part of the pilot bearing) and to form the bottom of the thrust bearing, instead of having the bearing surface thin and braced with triangular webs, which was going to be hard to make.












That process looked like this...

Chose some plate and a piece of hard shaft from the scrap bin. 

Little was needed to make the pin besides cutting to length, making a bevel on the end to aid assembly, and polishing it to a fare-thee-well, because it bears against a dry bearing liner of Nylatron GSM.  (no maintenance)

Did the usual things to make a square plate round.  Bored a very precise hole in the center, a few thousandths too small to admit the pin.

Heated up the plate until the pin dropped in and let it cool.  Never to be removed again, they are one piece now.  Could that pin be pressed out with a hydraulic press?  Maybe.  I don't want this part to come apart.

Machined the OD of the bottom bearing assembly to the ID of the mast.
The lip on the chuck side plus the close fit lets this sit in the top of the mast without fasteners or welds, meaning it's easy to service some day should the need ever arise.

This shows the general arrangement of the lower bearing assembly inserted into a scrap of mast tube, with a scrap of the square jib housing tube around it.  A large acetyle washer sits on the big flat part to make the lube-free bearing surface of the thust bearing. A cylindrical bearing lined with Nylatron GSM rides on the pin.  Both of those are part of the upper bearing assembly, which is bolted to the top of the jib housing, so that will be covered in Part 3: The Jib Housing.