Project 2
Tripod Support for Portable Masts

HOME
24/7/2013

---------------------------------------------------------------------------------------------------------------------------------------------------------

David Cleland VK7DC

A while back our club had a bit of a field day where Steve VK7BI brought along a plaster sheet lifter that he had slightly modified and fitted a mast on top. The lifter featured a tripod base. This was the spark for my own creation, a fold up, lightweight, tripod base that is sturdy enough to support a 12 metre aluminium mast and yet compact enough to fit in the back of my station wagon. Once assembled the base is quite capable of supporting my full weight. While the base is reasonably stable in its own right, the use of guy ropes would be recommended for long term use or in anything other than calm conditions. This tripod could easily be scaled up or down to suit user preferences.

<The Base.>

This base took me about a weekend to construct and the main tools required were an arc welder, bench drill (preferred), battery drill, angle grinder using both cutting and grinding discs, a set of six mm (or 1/4" BSW)  taps and, obviously, drill bits. The cost! It was less than $100. A tip, however, is to shop around for steel as prices vary considerably. For corrosion resistance reasons all the steel was purchased as DURAGAL finish where possible.

Construction

The base legs are 1.3 metres long and constructed from 25 x 1.6 mm square hollow section S.H.S. The lower brace sections of the legs are approximately one metre long and spaced apart from the top section by 200 mm where they affix to the upright.

< <The pivots.>

<The legs.>

The upright is 900 mm long, 40 x 2 mm square hollow section.

The lower leg brace sections also are at 90 degrees to the upright and therefore the upper section of the leg slopes down from the upright, raising the bottom of the vertical upright from the ground about 50 mm or so. The ends of the legs have a vertical 80 mm long section of 25 x 1.6 S.H.S. welded on to allow the use of a height adjustable extension to provide an amount of levelling.

< The leg ends.>  

The leg levelling sections are 300 mm long, 20 x 1.6 mm square hollow section S.H.S. with a loose fitting base pad made from 50 x 3 flat plate.

< Swivelling leg.>

The two folding legs are locked in place with a pair of lock bars 400 mm long, 30 x 3 angle iron. The bars are drilled with 6mm clearance holes at approximately 10mm from each end. The ends are bent in a vice so the ends sit flat on the legs where the bolts attach.

< The locks >

The lock bars are held in place with six mm diameter set screws. The securing holes in the legs are tapped six mm. Once the legs are attached to the upright, set up the frame on the ground and from corner to corner the base legs measure about 2.38 metres. Set the spacings between all leg ends so that they all measures equally. The holes for the lock bars can then be marked on the legs and then drilled and tapped to suit.   

The bottom mast clamp is mounted on a heavy duty gate hinge to allow the mast to be attached while it is laid down, refer Photo 7, and then walked to the upright position. The hinge is secured to the upright using a pair of heavy duty self-drilling six mm diameter metal screws, refer Photo 8.

<Photo 7: The heavy duty gate hinge.>


< The hinge secured to the upright.>

The top mast clamp is mounted through a 50 x5 mm flat plate which is secured to the upright in a sandwich fashion by clamping, using a pair of six mm diameter bolts. The front plate has six mm or ” BSW threaded holes. The bolts do not protrude though the plate. Refer Photo 9.

< The top clamp.>

Usage

The tripod base is set on the ground and the adjustment legs are then fitted. Fold the legs out to their approximate 120 degree spacing and fit the braces so that the legs stay at the correct spacing. If the mast is to be pivoted upright then make sure the fixed leg is pointing in the opposite direction to the direction the mast will be laid prior to erection. Adjust levelling legs if required. Put the base of the mast in the bottom clamp as shown in Photo 10 and then tighten the clamp.

< The bottom clamp.>

I find that the ‘U’ bolts really do not need to be done up more than I can tighten by hand. When the weight comes on to it the clamp twists downwards at an angle and binds. Being as the clamp is a bit wide for the tube mast it is not totally locked in line with the axis of the hinge when raising and lowering, thus preventing the hinge from laterally twisting and being ruined. Once the mast is stood up the top clamp is fitted to keep the mast upright. Realistically this is the weakest link in the erection process, as it is possible to drop a nut when attaching the top ‘U’ bolt. When time permits I will make some other type of latch that can be quickly slipped on and then just tightened up.

Without the tripod base being pegged or held down, I could just walk the mast upright if I limited its length to nine metres. The mast is made from three metre lengths of thick wall 50 and 60 mm aluminium tube. I have another lighter weight nine metre mast which walks up very easily and the base was given its first real use with that mast attached at an equine endurance ride where our club provided safety communications.





< The tripod in use.>
---------------------------------------------------------------------------------------------------------------------------------------------------------


HOME