28 May 2000
As I write this it's snowing outside. Big fluffy
crystals of frozen water are swirling, driven by the wind, in a sky
that was supposed to be blue and far too warm for snow. You
see last winter was supposed to be just that, our "last winter"
however the truck is nowhere near finished. Apart from moving house
and a couple of weeks of total motivation atrophy I have done
the following...
- Finished the cladding (well almost)
- Installed the stair raising/lowering buttons
and relays
- Mounted the hot water system (not connected)
- Installed the slide rails for the TV
- Built the main frame for the deck
That's something I suppose but in general
I've been a while getting back up to speed.
15 June 2000
Brrrrrr, minus 6 last night and with all the hype about GST I've
thought of some expansions of the acronym more appropriate for me.
- Although it's not as fast as I'd like the motor
home project is Getting Slowly There.
- I'm really Getting Stuckin Toit now
though.
- As soon as it's finished I'll Get Seriously
Tanked.
- Then hit the road and Goto Sunny Townsville.
For the past week or so I've been working on the
deck. "A deck!" you say. Yep, after a very wet Roma rally
where we saw most motor homers with sodden under-awning areas and
one with a lovely dry deck we decided that's for us.
Fig 1. The basic configuration.
Figure 1 shows the basic configuration where the deck stores
vertically at the rear of the rig, is hinged at the bottom and
folds down. Naturally this has to be lowered somehow and my first
impulse was to use gas struts in the usual manner something like
this...
Fig 2. Using a gas strut in the usual way.
The problem here is that the strut would hit the body when the
deck is raised so an extension would be required.
Fig 3. Adding an extension to clear the body.
Trouble is this creates a protrusion that people
will bash their head on and that may be damaged when exiting a
steep creek crossing.
My next thought was to use a winch, this would
probably have been the easiest method but they're expensive and
really noisy. Similarly hydraulics are expensive, noisy and a
little hard to install.
So I returned to gas struts.
What was needed was a way to use the struts
on the top of the deck but this would mean they have to pull up,
and that's not the way they're designed. So how about using a
cable to pull on the struts as follows?
Fig 4. Using two struts and a cable.
In this configuration two struts are ganged,
attached to a cable which runs over a sheave (pulley) to an attachment
point on the deck. This provides a lifting affect at a point about
200mm from the deck's pivot point. As the deck in about 1800mm
long the force required is about nine times the deck's weight.
As it happens this is about 500kgs so I duplicated this setup
on the other side of the rig giving me a total of four struts,
each of 130kgs force.
So far so good, but the trouble is that the
force applied by the struts is more-or-less linear but the force
required is not. As the deck is raised less force is needed to
hold its weight.
Fig 5. Less force need as the deck is raised.
As the deck approaches the point shown in Fig
5 you are pulling against 500kgs of gas strut (the deck is raised
and lowered from the ground below the rear of the truck). Naturally
you would loose and the whole lot would slam against the rear
of the truck when closing and be impossible to open in the first
place.
One answer is to design the cable to be of a
length that allows the struts to be fully extended when the deck
is at around the position shown above. At this point you don't
really need any help so it's a simple matter to go inside the
rig and pull the deck closed.
Fig 6. Photo of the struts.
Handrails
So we have a lowering deck but what's going to keep it level
and strong enough to hold people?
My first thought was legs. A couple of adjustable
legs positioned near the outer corners would do the trick but
I didn't want any obstruction under the deck and there's
also the fact that the legs would have to be stored somewhere.
I wanted the deck to be self-supporting. Enter the handrails.
As you have to have handrails anyway why not
make them structural?
Fig 7. The handrail becomes a structural member.
Here we see the handrail in place. It is hinged
to the deck at the bottom (blue line) and attached to the motor
home body (red arrow). This creates an effective steel beam 700mm
in height, more than strong enough to support a couple of people.
There is a rear handrail as well but this has
no real structural value. It just ties the two side rails together
and stops us from falling off.
Chains
To stop the deck at the correct level I have added to
high-tensile chains as follows.
Fig 8. High tensile chains added.
Safety
This deck is two metres from the ground, not far to jump on
purpose but a long way to fall by accident. As mentioned elsewhere
I'm a firm believer in redundant systems where there is a backup
in place for the primary device. With the deck this translates
to,
- The strut/cable arrangement holds the weight
as the deck is being lowered or raised. There are four struts
organised in two assemblies each with a steel cable. If any
part of either assembly fails the other will not hold entirely
but will slow the rate of fall to an acceptable rate.
- When the deck is deployed the main stress
is taken by the handrails. There are two rails and each one
can easily hold if the other fails.
- There are two high-tensile chains that backup
the handrails. As before each would hold the entire weight if
both handrails and one chain fails (well you never know).
The chains are welded to different locations than the handrails
on both the body and the deck, no point having a backup assembly
dependant on the same thing as the primary.
Fig 9. Top shot of the deck
Fig 10. Underneath shot of the
deck
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