Wednesday, July 9, 2014

Overhauling a BBMB Mast Bearing

The majority of rotatable yagis mounted on a tower require two plates (platforms) at or near the top of the tower to mount the rotator and a mast bearing. The bearing fits over the mast and transfers (lateral) wind forces on the mast and antenna to the tower while allowing the mast to freely rotate and greatly reducing bending moment at the rotator.

Since the bearing acts as a fulcrum to the mast's lever it is important that it be correctly placed when the mast extends far above the top of the tower to support stacked yagis. This is accomplished by moving the rotator plate lower in the tower. Either way the bearing is subject to extensive abuse from wind loads and precipitation. Eventually every mast bearing (sometimes called a thrust bearing around here) will require maintenance or replacement.

Many tower manufacturers sell mast bearings that mate to pre-drilled holes on the tower plate. There is a multitude of industrial bearings and matching flange housings that are cheaper and capable of being employed as thrust bearings but for the following shortcomings:
  • Weather - The bearings, while nominally sealed, are open to the elements.
  • Mast adjustment - There is no provision for a mast size other than the one that exactly fits the bearing's inner diameter.
  • Vertical force - Although the mast bearing is not designed for vertical forces it can deal with a modest amount of it. A typical industrial bearing has less ability in this respect since that is not its intended use.
Although the purpose-built thrust bearing tend to be expensive -- this is a low-volume specialty market -- they are almost always the superior choice. That doesn't mean we must always buy new, if you are willing to put in some effort.

The BBMB mast (or thrust) bearing made by Wade Communications (Delhi) for their DMX towers is typically sold by Canadian retailers for $85 to $90, plus tax and shipping. It is made from 2 pieces of cast aluminum that are held together by the ball bearings in the mated half-races, as shown in the diagram at right (taken from the spec sheet linked to above). There is a hole in the outer surface through which the bearings are inserted at the factory. A steel slug is sealed into place by pressing the aluminum of the cast body over the edge of the slug. This product is designed to be replaced not repaired.

Replace is just what many hams with DMX towers will do when the bearing succumbs to the effects of age. Yet it is possible to repair these sealed bearings if the internal wear is modest and it has not been abused by excessive loads or poor installation. Often it only requires cleaning and greasing the bearings and races. That is, if we can get inside to do it.

I know many hams who have repaired these bearings and I have done a few myself. A web search failed to turn up any instructions from hams who have elected to tell others how it's done. Since I was going to repair an old BBMB anyway it seemed to be an ideal opportunity to fill this gap. Although this procedure is specific to the BBMB it may also prove helpful to owners of other types of towers and bearings. It only takes an hour, or perhaps two hours if you're doing it for the first time.

Opening the bearing

The only difficult task in the overhaul process is the removal of the slug that seals the bearings inside the race. Until this comes out the bearings cannot be removed and the halves of the bearing cannot be separated.

To remove the slug it is necessary to grind or file away the aluminum from the casting that is holding the slug onto the housing. From what I've seen of these bearings there are 3 equally-spaces arcs of aluminum pressed in over the edge of the slug. Unfortunately the area is quite small and I couldn't take a picture (with my camera phone) that gets in close enough to show this. Look for it on your own BBMB and you'll see it.

A Dremel tool can be used or a small cylindrical grinder that fits an electric drill. Alternatively, if you have a small, curved file you can manually remove the aluminum. This is not precision work so don't worry if you remove more of the housing material than necessary. Just try not to enlarge the circular slot in which the slug rests.

If there is still resistance to the slug coming out you can drill a small hole in the steel plug and use a sharp, strong tool to lever it out. In this instance I did it with a steel awl with a hardened tip.

Don't lose the slug! You'll need it later.


Disassembly

You will need a small container for this next step. While holding the outer half of the bearing over the container slowly rotate the square flange. The bearings will, one by one, roll over the now open hole and drop into the container. You may need to shake or tap the housing if the balls stick to the grease residue.

This time around I counted the ball bearings: there were 29. There is room for more in the race so don't quote me on this figure.

You'll know when the last ball bearing is out when the halves of the bearing slip apart in your hands.

Cleaning

Pour a grease solvent into the container holding the ball bearings until they're submerged. I used the cheapest available: paint thinner. For this application there is no value in spending money on higher-quality solvent. Drop the slug into the container so that it also gets cleaned.

Dip a paper towel or disposable rag in the solvent and use it to clean the dirt and grease from both halves of the bearing race and other surfaces that are in close contact to the other half of the bearing. Some vigourous rubbing may be required.

When the parts are clean to your satisfaction wipe and dry the surfaces and ball bearings with a clean paper tower or rag. Letting them air dry may leave some residue -- this is a disadvantage of cheap solvents.

Inspection

When you look at the cleaned races you'll almost certainly notice that the races are indented, one indentation per ball bearing. This is to be expected with steel balls and an aluminum race. The same thing occurs in Hy-Gain rotators, though to a lesser degree.

If the indentations are shallow and smooth you can get more service life out of the bearing. If the aluminum is so badly galled or abraded that there are sharp edges or deep pits in the race it may be time to purchase a new thrust bearing.

Pitted balls can be replaced. Take one to a motor or bearing shop and buy replacements of the same size (I didn't measure them). It is (far) more likely that some balls bounce away during disassembly and you can't find them. It is also not unknown that the slug falls out of the housing on its own and steel balls rain down from the top of the tower from time to time. Yes, that really does happen with this bearing. At least that saves you the trouble of removing the slug!

Assembly

Assuming that the bearing passes your inspection you will need a few items to put it back together.
  • Grease - I use the same white lithium grease that I also use for rotators and protecting hardware. Use a liberal amount on the races and on the top and bottom rims of the lower housing (the one with the flange).
  • Slug - If the slug is damaged during disassembly you may need to replace it. If it's just bent you can hammer it flat again. If you do need a replacement keep in mind that it is smaller that any coin I have. The smallest in my collection is a pre-Euro 1 peseta coin (Spain) and it is too large. The slug should be steel but I think that any reasonably thick metal will suffice. Just be sure whatever you use to replace the slug does not penetrate into the hole since that will interfere with the balls' movement.
  • Hose clamp - A 3½" hose clamp (stainless preferred) to secure the slug after reassembly.
After reassembly test that the bearing freely turns, only showing resistance due to the grease. As the excess grease is pushed aside by the balls the resistance will decrease. Wipe off any grease that is pushed out at the seams between the bearing halves.

Also test that the bearing turns reasonably well with a modest amount of vertical force. Do this by placing the bearing on a flat workbench and turning it while pressing down with your hand. If there is metal-to-metal contact you'll feel it. A small amount of contact is not a problem.

If the resistance is significant you should disassemble the bearing and sand the surfaces of the bearing housing that show signs of contact. You can see in the cutaway view at the top of this post where this contact is likely to occur. Do not sand or file the races! Be sure that all metal filings are removed before greasing and assembly.

You can see that the bearing is not designed to turn under high vertical force by the fact that the the housing only behaves as a bushing to this force, with only grease to prevent binding. The balls offer little support against vertical forces.

To finalize reassembly insert the slug into the bearing hole and tighten the hose clamp around the housing. You may need a shim to hold the slug securely since it sits slightly below the housing surface. The final result should look like the one in the adjacent picture, which is the one I just finished overhauling.

Installation

You will likely need new hardware to bolt the flange to the tower plate and screws and nuts for the mast adjustment. The stock hardware supplied with the bearing is not stainless and readily rusts. Use a bolt cutter if necessary to remove old hardware; the old hardware isn't worth saving.

To mount the flange to the tower plate use 5/16" bolts that are at least ¾" long. You'll need a lock washer as well. The mast adjusting bolts are ¼" and should be at least 1" long, or longer if the mast diameter is small.  If you want to avoid excess length of adjustment screws protruding from the housing you can choose screws that are just long enough to secure the mast you're using. Two nuts are needed on each bolt: one on the inside to set the distance to the mast and one on the outside to lock the screw. High strength hardware isn't required, so use coated (and greased) or stainless hardware to inhibit rust.

2 comments:

  1. Thanks for sharing -ve3vv

    ReplyDelete
  2. Thank you. I did it and it works. I also used fine sandpaper to clean the bumps in the housing. VA3DIF

    ReplyDelete

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