How to calculate the hydraulic system requirements for a tipping trailer.

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1 How to calculate the hydraulic system requirements for a tipping trailer. Disclaimer: I am not an engineer or hydraulic expert. This info has been compiled from my own experiences designing and building a tipping tray on my trailer. This information may have mistakes or erroneous assumptions (if anyone can see and correct errors please advise me.) Use this info at your own risk. The short story: This write-up contains my limited knowledge on how to calculate the hydraulic requirements to lift a trailer deck, using a single stage, underbody cylinder. I have attached a spreadsheet that you can enter the parameters and it will calculate the hydraulic pressure and tipping times. To calculate the hydraulic requirements to lift a trailer deck you need to know: The cylinder internal diameter, calculated from the internal diameter. The position under the deck the cylinder will push on, relative to the hinge location The length of the deck. The total weight of the deck and load that you want to lift. My calc assumes an evenly spread load and no allowance for the deck overhanging the rear hinge point. The angle between the deck and the cylinder. The available hydraulic flow The cylinder stroke (flow and stroke determines how long it will take to tip) I struggled to understand how to figure out whether a tipping trailer I was building would work, so spent a lot of time researching, and was surprised at how little info specific to this subject that I could find. I got some advice from others so I decided to contribute what info I have gathered so others might get some use. I wanted a dual purpose tipping trailer that could be towed behind my tractor, and tipped using the tractor external hydraulic ports, or towed behind my small truck and tipped using a petrol driven hydraulic power pack. I did not want an electric 12v power pack because previous experience showed that these wreck vehicle batteries if used more than once or twice a day. I think the batteries overheat. Don t let the 12v power pack salesman tell you otherwise. It cost me hundreds of dollars in batteries before I ditched my electric/hydraulic power pack on another vehicle a few years ago. Cost is always a key consideration for me so I collected and reused parts. The trailer frame, suspension and wheels came from a 3 ton boat trailer. The tipping deck came from a dismantled truck. The double acting hydraulic cylinder came from a dismantled Ford 5000 backhoe.

2 Diagram 1 above shows the basic design of the tipping deck. Picture 1 above shows the resulting trailer I built.

3 I ll start by discussing the set up and calculations for the hydraulic cylinder required to tip the deck. Inityially I designed and built the trailer to be tipped by the tractor hydraulics. I added the petrol driven power pack later, so I will discuss the calculations for the power pack afterwards. Because I got the cylinder free I designed the trailer to suit the cylinder. If you re starting with nothing, the first thing is to understand is the types of hydraulic cylinder configurations used for tipping decks. Here s a few of the common ones. Front of body (FOB) - multi stage / telescopic cylinders Probably the easiest to calculate and install, but requires a cylinder that extends a long way which might cost more. Also depending on the design you might have a cylinder sticking up above the front of deck when it is down, as shown in this pic. OK for trucks, but maybe not OK for a trailer. Above: typical FOB tipper. Underbody single stage cylinders This is what I built and this article and associated spreadsheet will show you how to calculate the hydraulic requirements. Underbody multi stage / telescopic cylinders Possibly easier to fit the cylinder in under the deck, and build if you want to buy a suitable cylinder. Because the retracted cylinder is short these can push almost directly upwards under the trailer.

4 This set up seems to be fairly common for trailers. Above: typical multi stage / telescopic under body tipper Underbody scissor lift cylinder Usually bought as a kit or prebuilt unit. I haven t investigated these types so have no advice about them. Above: Scissor lift underbody hoist

5 Underbody single stage cylinder design In my experience you need to do the physical design and hydraulic calculations many times before you finalise the solution. It s worth spending lots of time going repeatedly going around this process because it saved me building the wrong design that would not have worked. The process I went through was to: 1. Draw the layout to scale on paper like in diagram 1 above. 2. Calculate whether the hydraulic requirements could be met with my tractor 3. Mock up the layout on the trailer frame using lengths of timber, and measure the actual lengths and angles when simulating the raised and lowered position. I was a bit surprised to find that the actual measurements never exactly matched my paper design, so don t miss this step. 4. Re-do the drawing and calculations based on the actual measurements. Go around this loop until you get it right, then start building. Forward vs rearward facing cylinder. My design in Diagram 1 above shows a forward facing cylinder i.e. the part attached to the trailer frame is at the rear, and the part attached to the tipping deck (which moves up) is further forward. The photo below shows a rearwards facing cylinder. Either layout can be made to work with the right design and components. I started out designing with a rearward facing cylinder, but with helpful advice from redman on tractorbynet I soon learned that with the size cylinder I had and the tractor producing 2250psi (150 Bar) hydraulic pressure that it was better to use a forward facing cylinder. Hopefully I can explain why later in this article. Above: Trailer with rearwards facing cylinder. Notes on terminology Ram vs cylinder. We often refer to hydraulic rams, but I found that the experts call them hydraulic cylinders, so that s what I have called them here. I believe they are the same thing. The part that extends and retracts is called the spear. Double acting vs single acting cylinder. A single acting cylinder has 1 hose connected to it. Pressurised oil goes into the cylinder via the hose and pushes the spear/telescopic cylinder out. You rely on gravity/external forces to retract the cylinder. When you move the valve to the lower or retract position oil flows back through the same

6 hose via the valve to the oil tank. A single acting cylinder is the most common for tipping trailers. Usually you are only concerned about the hydraulic effort required to push the tipping tray up. Getting it down is seldom a problem as it can come down under its own weight. A double acting cylinder has 2 hoses connected to it. Inside the cylinder there is a piston with oil seals, and the spear is connected to that piston. With the appropriate control valve the oil flows in one hose and pushes at one side of the piston/spear. At the same time the valve allows oil to come out of the other end of the cylinder and return to the tank. When the valve is reversed, the piston/spear travels on the opposite direction. I got a large double acting cylinder from a dismantled Ford 5000 backhoe for no cost, so that s what I used. It s fairly obvious from this description that you need a valve to suit the cylinder type. Perhaps another time I will write up what I learned about different types of valves. Gallons. I will not use gallons or GPM as a unit of measure. The reason is that US vs Imperial gallons are different. 1 US Gallon = litres Where as 1 Imperial (UK) gallon = litres That is about 20% difference. I think it is almost a crime that most websites and suppliers info does not specify US or imperial when talking about gallons/gpm. So you don t actually know what capacity they are actually talking about. Petrol Petrol is the liquid hydrocarbon mix that we run most car engines on. In the US they call it gasoline or gas. I am surprised the Americans can t differentiate between matter that is in a gaseous state or liquid state. I also found the US confuses things further by referring to a gas engine that runs on LPG. I know there are a lot of US members on this forum but I will still call it petrol. How much weight will the cylinder have to lift. When I started this project I naively thought the cylinder would need to push a total of 3500kg because that was the maximum weight of the tray and load. I soon learned that this is not correct because leverage gets involved due to angles. Calculating the weight to be lifted. First thing you need to know is the total weight of the tipping tray and its load. In my case I wanted to carry 2500kg (5500lbs). The re-used deck I had weighed 250kg. Since part of the work was carrying dirt I wanted to allow a margin for error as we always seem to underestimate how much dirt we have loaded on (just one more bucket full will be OK), so I designed it to tip 3500kg (6600lbs) total. Next thing to figure out is the position of each end of the cylinder mounted under the deck. You need to determine where under the deck the cylinder will be pushing upwards, and how much angle there will be between the cylinder and the deck. When working this out you need to plan out where the cylinder sits in the retracted (deck down) and extended (deck up) positions. I found that fairly small forward/backward adjustments to the cylinder position made quite a big difference to the maximum tipping angle. To get the greatest possible angle between the deck and cylinder with the deck down, I mounted the bottom of the cylinder quite low to the ground and as you can see in the photo, it ended up very close to one trailer axle. This is where the frontward facing vs rearward facing cylinder comes into play. Initially I designed the cylinder to be rearward facing so it wasn t getting close to the axle, but the place under the deck where the cylinder would be pushing upwards was too close to the deck

7 hinge and due to the leverage, this reduced the amount it could lift. My cylinder had a retracted length of 1180mm pin centre to centre. Fully extended it was 1960mm so has a stroke of 780mm (78 cm). In my case I figured out that I could get 22 degree angle between the cylinder and deck. The cylinder would be pushing upwards 960mm from the front of the deck (or 420mm from the centre of the deck). I ll talk more about the cylinder position a wee bit later. Tipping angle. Many comments on the internet recommended a tipping angle of 55 degrees. I did a test with a small truck and found that loose gravel or dry dirt or tree branches tips off between 42 and 46 degrees. Wet clay and mud tipped off at 46 to 50 degrees. So I would recommend a minimum of 50 degrees. The trailer I built actually achieved 60 degrees. Cylinder diameter and volume and pressure. If you have a single acting cylinder the effective working area is the diameter of the spear. For a double acting cylinder the working area is the internal diameter of the cylinder. My double acting cylinder is 8.9cm inside diameter = 4.45 cm radius. The area that the fluid pushes on is calculated with formula Pi x (radius x radius) this can also be written π r 2 Pi is near enough to so the calculation is x (4.45 x 4.5) = cm 2 (9.644 sq in) This is multiplied by the hydraulic pressure to calculate the maximum force the cylinder can push directly out the end. My tractor in theory produces 155 kg/cm 2 (about 2250 psi) so: cm 2 x 150 kg/cm 2 = 9644 kg In imperial that is sq in x 2250 psi = lbs Obviously, if you have more hydraulic pressure you get more lifting force from the cylinder. BUT REMEMBER: Since the cylinder is on an angle it is not pushing the deck up with that much force. Now we have to use a trigonometry calculation to figure out how much vertical lift will be applied to the tipping deck. The calc is sin angle between the cylinder and deck x cylinder force sin 22 deg x 9644kg = x 9644kg = 3613kg vertical lift. You can get the sin of an angle using some of the more advanced pocket calculators, a spreadsheet or ask google e.g. what is the sin of 22 degrees. The key point is that if the angle between the deck and cylinder is smaller, then you get less vertical lift. Length of the deck and deck overhang To simplify calculating the tipping load capacity I ignored the overhand behind the hinge point. In the diagrams below you will notice I have not drawn the overhang. Any weight you put on the overhang acts as a counterbalance and makes the tray easier to tip so that gives you more leeway for error in your calculations. Remember that I am always assuming a load evenly spread across the deck. Where is the cylinder pushing under the deck? You have to figure out how much force will be required to push up the deck at the point your cylinder connects to the deck. The amount of force required to push up will be a little bit more than the

8 weight pushing down at that point. My way of understanding this was to envisage the deck with the load lifting point at various places. I struggle to explain this well but here goes KG Front 1750 KG Hinge 1750 KG Diagram 2 above: Load carried on front and rear Ref. Diagram 2 - If the cylinder was pushing at the very front of the deck, and hinged at the rear, then the load would be shared equally between the 2 points. So my total load of 3500kg would result in 1750kg on the hinge point ant 1750kg on the cylinder. Note that this is scenario you get with an FOB hoist KG Centre 3500 KG Hinge 0 KG Diagram 3 above: Load carried at centre (balance point) Ref Diagram 3 - If the cylinder was pushing on the centre (balance point) between the hinge and front of the deck then there is pretty much no weight on the hinge, and the cylinder has to push the full 3500kgs upwards.

9 3500 KG Load point 2967 KG Hinge 533 KG Diagram 4 above: Load carried forward of (balance point Ref Diagram 4 As you move the load point that the cylinder is pushing away from the centre (balance point) of the deck towards the front of the deck, then a leverage effect decreases the amount of push required from 3500kg to 1700kg. I assume the reduction in push required was proportional to the distance moved towards the front. In my case the cylinder is pushing 1715mm from the hinge point which is 1045mm from the front. The distance from the centre to the front is 1380mm. I calculated the load as follows: 1750kg + (960mm/1380mm x 1750kg) = 2967 kg At this stage I was a bit uncertain about whether this calc was correct, and it is fairly close to the centre point load of 3500kg, so I used 3500kg for my calculations. If anyone can confirm/correct my theory it would be appreciated. Earlier in the article I calculated that at 22 degrees angle the cylinder could push a vertical lift of 3613kg. So it would work with a margin for error. The other situation you could encounter is if the cylinder was pushing somewhere between the centre (balance point) of the deck and the hinge point, then leverage increases the amount of push required to lift the deck. This is what would have happened is I built it with the cylinder facing rearwards. That was my original design and the lift point would have been 960mm from the hinge point (420 mm behind centre point) so would have required 4033kg to start tipping. As you can see, this would not have worked with the hydraulic pressure my tractor produces. How long to tip? The last piece of the jigsaw was how long it would take to tip the tray right up. With my tractor this was not a concern, because I know it has a fairly large volume hydraulic pump (although not particularly high pressure). However, it was worth figuring out as I would then use the same formula for the petrol driven power pack I am adding in the second stage of the project which is work in progress. Earlier I calculated a cylinder area as square cm. Multiply cylinder area square cm by the cylinder stroke of 78cm gives a stroke volume of 4853cc (4.853 litres). With the pump delivering 38.6 litres per minute (at 155 bar),the time to tip up is:

10 Stroke volume (litres) / Pump Flow (LPM) = minutes to tip right up 4.853/38.6 = minutes i.e. about 7.5 seconds for the tractor hydraulics Adding a petrol/hydraulic power pack Since my tipping trailer is built with 2 quick release couplers to plug into the rear hydraulics on the tractor, I figured the same connectors could plug into a petrol/hydraulic power pack, so that I can take lighter loads of stuff on the public roads towed by the ute and unload it without lots of hard labour. I also wanted to take the trailer to my brothers place and use it behind his tractor, which does not have external hydraulics. I already have a near new 4.8kw / 6.5 hp electric start Honda clone petrol engine left over from another job, so that was tagged for my power pack. I wanted to run it at no more than 4.5 kw (6 hp) so it didn t risk overloading and stalling. I was allowing for the possibility that the Chinese exaggerated about the engine power output. I already knew that I needed at least 155 bar (2250psi) pressure to tip a full load. For reasons I don t quite remember, I decided to base it on 170 bar (2500psi). So I needed to calculate the flow I could get from 1 4.5kw motor producing 170 bar pressure. Once I knew the flow it would be capable of producing, I could select the closest suitably sized pump displacement and then I could calculate the resulting tipping time. The calculation is (Power available x constant) / pressure required = Flow But you need to then reduce this by about 15% to allow for pump inefficiency (4.5 kw x 600) / 170 =15.9 LPM 15.9 LPM 15% = 13.5 LPM The honda motor will run the pump at 3600 rpm, therefore 13.5 LPM / 3600 = litres (3.7 cc) per rev. The pump with nearest size down to this was 3.6cc/rev which gives a flow of 12.9 LPM I calculated above that the cylinder stroke volume was litres so at 12.9 LPM the tipping time is 0.38 minutes or 22 seconds. That is a relatively slow but better than off loading by hand. Why use a Dual stage pump? This discussion only applies if you are using a double acting cylinder. When I went looking for a pump about 3.6cc/rev and capable of running 3600rpm and 170 bar I found some moderately priced single stage pumps called model HGP-xx that could do the job. My internet searches also found many 2 stage pumps being sold for log splitters. These are clever little pumps that automatically switch between low pressure/high flow and high pressure low flow and they don t cost much more than a single stage pump. They got me interested because I had a double acting cylinder and with a single stage 3.6cc pump it meant it was going to take as long to come down as it was to go up. i.e. 22 seconds. The 2 stage pump I selected would produce 12.9LPM at 170 bar that would get used when pushing the tray up. When the tray was coming down there would be very little pressure required (maybe 3 bar) so the pump would run at 39 LPM and it would only take about 6 seconds to come down. If you look closely at the spreadsheet I provided you will see I need to allow for the reduced stroke volume for the side of the cylinder that has the spear in it. This works in my favour by reducing the going down time.

11 If you re using a single acting cylinder and appropriate valve the tray descends under its own weight, and the time it takes is dictated by how quickly gravity can push the oil back through the pipes so it is normally fairly quick going down. Finally here is an alternative way I found to calculate the motor size. If you know the flow and pressure your tipper requires then the calculation to find out the size of motor required for the power pack is: Flow LPM x Pressure Bar / Constant 600 = Power in KW, then Divide the result by 0.85 to allow for 15% pump inefficiency. For example: (15 LPM x 200 bar) / 600 = 5 KW, then 5 KW /.85 = 5.9 KW minimum power required (about 8 HP) I recommend you buy a motor that is a bit bigger than this. This example is not what I did for my power pack. About hydraulic valves. There are plenty of good web sites that describe how valves work for double acting or single acting cylinders, so I won t go into that here. However I did discover a couple if interesting thing worth knowing. On the face of it a log splitter valve appeared to be ideal for my double acting cylinder on the tipping trailer. They had all the right functions, were readily available, cheap, and the hydraulic schematics said they should work. Many of them also have a detent that log splitters use when going back on low pressure. The ideas is that the operator pushes the valve to the detent position, then lets it go. When the cylinder fully retracts and stops moving the pressure suddenly rises (2 stage pump) and the detent pops the valve to the middle neutral position. I was warned by a hydraulic supplier that on my trailer this arrangement can lead to a jerky motion when the tray is coming down. Something to do with the weight of the tray overtaking the pump pressure and causing wild fluctuations in the hydraulic pressure. He convinced me to spend $15 more on a valve that was better suited with no detent. This situation only applies to double acting cylinders. Before buying your valve, carefully plan out where the valve will be mounted, and how the hoses will connect to it. If you can plan it so the hoses all have straight hydraulic hose ends you will save money because 45 /90 degree ends and/or adaptors soon add quite a bit of cost. Valves have connections in different positions depending on brand and design.