< meta name="Description" content="Amateur Radio, Adapter plates, Yaesu to ROHN adapter plates"> Creating Adapter plates for Yaesu Thrust Bearings to ROHN 25G accessory shelves.

Creating Adapter Plates

Do not consider this as directions for a Do It Yourself project, but things that worked for me and others may find helpful. Proceed at your own risk.

SAFETY NOTE: Adhere to proper safety practices, do not use machine tools unless you are thouroughly acquainted with and are proficient with their use. Wear proper eye protection. Do not wear loose fitting clothing, or long loose sleeve shirts that can become entangled in the machinery. Do not work alone.  Within the last few months a local, experienced worker was killed when his clothing became entangled in moving machinery.  I will add that some years back I lost the very tip of my left thumb when cutting a large sheet of plywood using a table saw by myself.

One of the problems of building things, Errecting towers, or just home projects in general and  not just ham projects is discovering we are lacking proper equipment, parts, parts don't fit, or we are trying to fit a metric part into or onto something using the American system of measurements. IOW it sometimes seems like a conspiracy against us ever completing a project.  I have to admit these projects have always gone much smoother than trying to do my own plumbing. <:-))

I run into this almost on a daily basis  as I like to work with towers and antennas. Often the work is brought about because of a need for maintenance which was caused by all sorts of things in nature and no too few times due to a miscalculation on my own part.

This project was brought about a couple of years ago when a flock of Cormorants decided to spend the night on the 7 element, 6-meter Yagi located at 115 feet on a mast 15 feet above the top of my ROHN 45G tower between the tribander at 100 feet and the cross boom for a pair of 11L 440's and a pair of 12L 144's..It's difficult to tell which antenna is located where, but the one just a bit above my head and the only one with a boom truss is the 7L 6-meter Yagi. The  weight of the birds plus the repeated landings broke the boom truss which then fell down across the driven element on the tri-band beam just above the top of the tower. This prevented using that antenna as well as rotating the whole works. I no longer climb as much or as long as I usee to, so it took some time to get the antennas, mast and rotator down.  I repaired the 6-meter Yagi and the tri-band beam. I sold the repaired tri-bander, sold it and p[urchased a used Force 12 C19XR Tri-bander.

I am in the process of installing a 50' ROHN 25G tower on the West end of my shop. I welded up a Dirt Baseand built a bracket (Include photo) and currently have 30 feet of tower installed.  The next section up will have a 24 foot lenght of 2" 6061-T6 Aluminum mast with a 1/4" wall installed in it. This mast will eventually be raised into the top section with about 10-12 feet extending above the top of the tower.  The tribander will be mounted just above the top of the tower at 50 feet and the 7 element 6-meter Yagi will be mounted about 10 feet above the Tribander.

This takes us to the point of this project.  The mast will be held in place by two thrust bearings during construction. When finished the mast will set in the rotator 12 feet below the top of the tower with a thrust bearing at the top of the tower.  The thrust bearings are Yaesu, while the ROHN accessory shelves that hold the rotator and thust bearings are drilled to accomodate the "Ham series" of rotators. The Yaesu Thrust bearings are of metric dimensions, while the accessory shelves are of the American dimensions.  IOW the bolt holes do not come near aligning to allow mounting the Yaesu thrust bearings.  This meant I had to design and build some sort of adapter to allow mounting the metric bearings on the American accessory shelves.

This is not nearly as complicated a procedure as many make it out to be according to the questions I see asked.

You start with the two parts and the material to be used as the adapter:

The ROHN Accessory shelf  on the left and a 6" X 8" X 1/4" 6061-T6 Aluminum Plate. Note, I happen to keep a few of this size plate on hand as they also make very good boom to mast and boom to element clamps for antennas.

The Yaesu thrust bearing showing the 8.5 cm bolt spacing.(M-8 bolts)

Now I need to find a center location on the accessory plane from which all other locations are based.

I placed the Aluminum plate behind the accessory shelf and traced the bolt pattern onto the plate. Transfer punches would do this more accurately but are not comm on in most tool boxes.  Now I need to locate the reference center.

NOTE, The normal approach at this point would simply be to take a plate large enough to encompass the larger bolt circle, punch the center and then using a compass, Measure the distance for the rotator bolt holes (center to center) scribe the circle on which the outer bolt hole lie. Then scribe the (in this case) the 85mm bolt circle and mark off 90 degree increments for the bolt holes to mount the bearing..Bolt the plate to the thrust bearing. Then bolt the adapter plate to the accessory plate using nuts for spacers. There needs to be enough space for the bolt heades to the thrust bearing to clear the accessory plate. Use a hole saw to open the center to roughly 2 1/4 inches. Center this on the same mark used for the compass when the scribe lines  (bolt circles) were created 


Using a straight edge and scribe I mark the lines centered on the bolt holes for the rotator. This could have been done a bit more accurately using transfer punches, but this will suffice.

They are faint but the crossing scribe lines can be seen at the center.  All work will be referenced to this crossing mark which should be center punched for permancence.

Note the bolt holes in the thrust bearing are 8.5 cm or 85 mm center to center.or 4.25 cm from the center.

The next step is to drill out the holes that match the rotator mounting holes. In this photo they are t The oblong marks.

The bolt holes for the thrust bearing are drilled on the same lines, but 4.25 cm from the center.  Note they do not necessiiarily have to be drilled on the same lines. They could be located half way between the lines or where ever is handy. I chose to use the same lines as it gave me a more rigid attachment even though the holes only catch part of the accessory plate.  NOTE when doing the drilling the plates should be securely clamped in the drill press.

The original plate, accessory shelf, and drilled out accessory shelf at the lower left.  The plate is bolted to the Aluminum plate through the rotator mounting holes, then the holes for the thrust bearing mount are drilled.  If you look closely you will see the two bolt holes closest to the inner opening on the lower left are only partially in the steel of the adapter plate.  Any rotation of the bolt hole pattern would have left one bolt only attached to the Aluminum, which would have been acceptable. Unfortunately the bearing is large enough to interfere with any bolts installed in the original rotator mounting holes, so this gives the most mechanically sound arrangement.  However it should b noted that this greatly increases the need for sufficient and proper claming of the work piece in the drill press for safety. A drill bit catching on the edge of that steel could send that plate flying with the possibility of serious injury.  NOTE again that the holes are drilled after the two plates are securely bolted togather. Note also that the Aluminum plate is now on top of the acessory shelf rather than underneath as it was when I first started. 

I could have also used spacers to raise the Aluminum plate away from the accessory shelf. Then seperate bolts could have been used to bolt the adapter plate to the shelf and the bearing to the adapter plate.  IOW there are a number of ways this can be accomplished.  I chose what was to me, the most simple and rugged...with the least work.

Please NOTE The measurements you come up with may differ from the ones I came up with.

However;  If you have a scribe, compass, and accurate ruller you can measure the distance between centers which we did for the thrust bearing and gound it to be 8.5 cm or 4.25 cm from the center/radius.  If you measure center to center on the elongated holes in the accesory shelf you will find  a distance of 4.5" or 2.25" radius.  Take a square plate large enough to cover the elongated holes by an extra half to one inch. Center puch the center and use that for the pivot point of a compass. Scribe two circles, one with a radius of 8.5 cm and one with a rasius of 2.25". Scribe a line from one side of the square to the other with the straight line passing through that center mark. Using a square, scribe a line at right angles the the first line. If the metal is square, the lines used to find the center will also work at this point. You can use either circle, but not both for the first set of bolt holes. You might find it a bit easier to use the inner circle at this point.  Again, using the center punch, punch where each line crosses the circle you chose. It would probably be most convenient to locate the outer set of holes midway between the inner set although its not critical. However IF the thrust bearing bolt circle is close to the sice of the outer circle the holes will need to be off set to clear the bolts in the other circle.  Locate the holes just as you did for the inner circle. You could also place the plate against the accessory shelf and line up the larger circle in the elongated bolt holes and punch on the circle for the bolt hole location.  Use some inguinity as there are a number of ways to find the bolt hole location. Just use what ever is handiest for you. I line up with the elongated holes and then position the inner circle holes (thrust bearing) mid way between the holes in the outer circle.   Don't forget to scribe a circle for the hole in the center large enough to clear the mast. This circle is more of a reminder than something necessary.   I used a 2" diameter hole saw to cut the hole for the mast. A milling machine could do a much cleaner job, but it would add nothing to the functionality.  OTOH those rough edges can be a bit hard on hands.

I'll try to get some photos up of the more direct method using the circles.

The entire unit assembled with the bearing bolted to the shelf through the adapter plate.  The center hole has yet to be drilled out.  I'll use a hole saw although a milling machine would do a much nicer looking job.  The purpose is to show the entire procedure can be done with normal tools most any one would have in their shop.






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