Today’s goal was to determine maximum pressing rates attainable with The Liberator, the open source CEB press. We obtained data to verify some of the predicted rates shown in italics in this table.
To boost morale, we started by pressing thin, 2 inch bricks (2″x6″x12″) – as their rate of production is higher than that of 4″ (4″x6″x12″) bricks. The thin bricks are the size of bricks one would press for making CEB floors. We ran the machine dry (without soil) in order to obtain baseline data. Results from runs with soil should be 5-10% lower if our calculations are correct.
This test run yielded 13 bricks per minute with LifeTrac as the power source, for the 2″ tall bricks. See this insanity in this one minute video:
Our results from today’s runs are incomplete but sufficient to move forward. Due to practical considerations on LifeTrac pressure relief settings, we ran into the limit of pressure valves beginning to crack open when running in medium-high throttle. This means that we could not obtain any meaningful data on pressing rates at the upper power limit of LifeTrac, so we may need to return to this analysis in the future.
On idle, with about 7 gallon per minute (GPM) hydraulic flow, we got 7 bricks per minute (for the 2″ bricks). Thus, the pressing rate is about 1 thin brick per each GPM of input. This value does not scale with GPM, however, because at higher flows, we actually need to slow down the thin (2.5″) soil-loading drawer cylinder, or otherwise it would move too fast and possibly cause damage over time.
The thin cylinder produces complications in the control scheme. Not only do we need to reduce the flow going to this cylinder – but also – we need to have a different reduction ratio for the expansion and contraction of the thin cylinder (given the design of a hydraulic cylinder) . This may be accomplished by using a check valve and a needle valve, in parallel, within the hydraulic circuit for the thin cylinder:
To make the machine work, one does not need to account for this complication – but if one is optimizing the machine, such details are important. The difference in the results could be up to 3 full bricks less per minute if one doesn’t account for this detail. This may not seem important with a machine of a 10 brick per minute or higher capacity. However, if one is streamlining operations or building cities – the difference between a 10 brick and 13 brick machine is over 1400 bricks per day. This translates to at least $1000 in productivity lost per day while using an unoptimized machine. We’re talking about serious production with The Liberator – so in the long run – every brick counts.
In any case, now we are doing better than our initial experience with our first, manually-loaded machine, which produced 1/2 brick per minute per person – because we did not know at that time about the difficulties in CEB operations lie in soil handling. Now – if one person loads the machine with a tractor front-end loader – that translates to a maximum of at least 12 bricks per minute produced according to the existing results. I must emphasize that this result obtains without the back-breaking labor.
While our results are incomplete, we still expect the 12-18 maximum as suggested in the former table, where 12 corresponds to full bricks, and 18 corresponds to the thin bricks.
Thus, what is the absolute physical limit for a simple, gravity-feed machine like ours – with a single compression chamber – pressing from the bottom – where the machine produces one brick at a time in a sequence of non-overlapping steps? The limits are determined by 3 factors:
- Physical speed of the hydraulic cylinders.
- Velocity of soil free-falling to fill the compression chamber.
- Speed with which the electronics can respond to the position sensors in the automatic control scheme.
The speed of the electronics is under 7 milliseconds for a detection loop, so it appears that this factor is not as important as the previous two factors – if the motion is on the order of 1 second per half-stroke for both cylinders. Thus, the physical limit will be a balance of cylinder speed and how fast the soil fills the compression chamber by free-falling from the soil-loading drawer and hopper. For the soil to fill effectively, it is reasonable to assume that each half-stroke for each cylinder is at the shortest about 1 second long – slightly faster than shown in the video – or about 4 seconds for the entire pressing cycle. This would be a theoretical value of about 15 bricks per minute – which is consistent with what we’re approaching in reality. In reality, we are approaching a main cylinder speed as fast as gravity is able to fill it.
Thus, we propose that once The Liberator reaches such a maximum limit or close to it, our work on liberating the means of construction is done. If the machine works as expected, there’s no need to make it any more complicated, and we can then focus on releasing the next product.
Others suggest that a horizontal press machine is a better choice than a vertical press like ours because the former can provide better uniformity in brick height. We will have to determine if this is a problem in our case. One immediate solution for our present machine is laying our bricks on their side. How much does brick-laying orientation matter? Another easy solution is to modify our machine to produce 6″ tall bricks (right now the absolute limits are 2″ to 5″ in height) – such that the other 6″ dimension is perfectcly uniform. I hope we never have to go the route of the horizontal press, due to the added complexity and cost. Our initial reason for choosing the vertical press was that it allows for the absolute simplest machine design. Correct us if we’re wrong.