I discover the next problem at home, my dad works with a transpalet and he has a bump in the main door of the garage. Usually when he’s working, he has problems to leave o enter with the transpalet, because the high of the machine is very limited. In the next pictures we can see the transpalet over the bump, it have to be in his higher possition to avoid this problem.
But when the transpalet is taking a palet, is impossible avoid this problem even in the higher possition, because the palet touch the ground (the bump).
This problem made me think about why his transpalet have this high and not another higher. Later to be searching about that, i discover that the main problem is the stability of the machine while it’s moving. If you have a higher charge gravity center, the stability is worst.
Then i decided to check that problem and see the difference between diferent possitions.
For that reason, i design a transpalet, using SolidWorks. Later, i connect the surface with a bump and the transpalet, using Adams, and simuling the process of his movement over the bump to see the interferences during the process.
The main objetive of my project is to check the stability of the transpalet. To get that, i have to create a rectilineal movement over the surface and over the bump, and control the velocity and position changes in function of the height and the velocity of the transpalet.
I could predict that the main problems will come with the simulation process, trying to reach the most realistis situation.
The modelling problem
For model, the transpalet has been simplify for the high complexity of the system. First, we have to decided what transpalet we are going to model, because there are different sizes, weights, and heights. Later to searh the best one, i decided to create my own model with the numbers of the highest transpalet. And i simplify the mechanical system of the machine. I ignore the pneumatic piston force, and i did it fix, becuase of the complexity of the system. I also simplify the mechanical system for the wheels, back and front, because if we would consider the tread function, it would increase so much the simulation and design problem.
With all of this simplifications, i’m trying to create a transpalet as near as possible to a real one, but considering some parameters fix. Mainly becase our objetive is to check the stability of the system, not all the functionalities of this.
Starting with the modelling, i design the next model:
Created and design using SolidWorks, 7 parts to create a simplify transpalet of 10 pieces.
Later i create the surface in Adams and i add the transpalet as parasolid:
When you introduce a parasolid in Adams, you have every solid independet, and you have to use connectors to create the movement relationship between them. My transpalet has 7 parts, formed by 10 different pieces. One charge body, one direction body, two connector for the two front wheels, one connector for the two back wheels, and one cylinder to connect all the back parts. Does not matters if you use more solid components or not, because using connectors you obtain the same result whitout add another solid. For that reason my design is more simple.
This is the complete design:
Now i explain the connectors:
I used two types of connectors for the model, fix joints and revolute joints.
The fix joints are being used to connect the cylinder with the base, the direction with the base, the connector of front wheels with the base, the connector of back wheels with the base and the main surface. And i used revolute joints to create the rotational movement in the back and front wheels, around his own longitudinal axis and with their connectors. We can differenciate two, one for front wheels and another one for back wheels.
Now that we have the connectors, we have to add motion where we want to create it. In our case, just for the rotational joints.
I created two motions, the same for each sistem of wheels:
We are looking for a motion as near as possible to the normal work movement where that type os transpalet are used. But i need to reach the limit point of use, if i use a velocity really slow i will not find the stability problems because i am working in his work range. For that reason i consider a velocity a little bit higher, to check what would happen in that situation for different height.
The velocity es different in each wheel, because they have different radius. Back wheel radius is 120 mm, and front wheel radius is 80 mm.
Later to simulate the behaviour of the system for different velocities, i decided to do the final simulation with the next function: 100.000.0d*time. That express a velocity betwenn 13 km/h or 14 km/h. The velocity of a person running. I know that this velocity is never achieve in a normal work diary, but i’m looking for the loss of stability, and with that velocity is easier to apreciate the difference between differents heights.
The last part of the modelling part is create the contact between the tires (cylinder) and the road (surface with bump).
Now i create the contact between the cylinder that works as road, because in the real case also is used a steel cylinder.
I create one contact for the two bodeis of the front wheel with the four solids of the surface (first solid is the first planar body, second solid is the first bump ramp, third solid is the second bump ramp, and fourth solid is the second planar body). And the same with the other contact, between the two back wheels and the surface.
For the contact i configured the next parameters:
Now i have all the configuration ready to start to simulate, but i can not yet because when i introduced the parasolid systems it was without mass, material and centre of mass marker. For that reason i define the mass by the geometry and the material type. That is Steel for all the components. Like the mass introduced for the program are so high, i choose a total weight of 90kg for the transpalet, that is a normal weight for a machine with his dimensions. The distribution of mass for all the components is:
And now i have everything to start with the simulation process:
Simulations and analysis of results
I am going to do three simulations, with the same conditions but different height, the first one with the lowest position possible, the wheel front angle is zero. The second simulation is in the middle height point, the wheel front angle respect the horizontal is 45º. And the third simulation is the highest possition, the wheel front angle is 90º respect the horizontal line. With the velocity that i already explain, and the next simulation configuration:
Here we can see the three situations at the same point of the simulation, more or less, and we can appreciate the difference of movement. Now i am going to explain each simulation as good as possible.
I analyse the velocity of the front wheel, the velocity of the back wheel and the center of mass position in the axis Z of the base.
Simulation 1 – 0º wheels
With the parameters and simulation characteristic that i already describe, i got the next result:
In first place, we see the velocity of the front wheel, like we introduce a lineal function, we have a lineal behaviour, but we can see the moment where the front wheels loss the contact with the surface, and this is during the descent of the bump. In the next graphic, we can see the same situation but for the back wheel:
And in the next graphic, we have to know first what represent. It’s the position of the center of mass in function of the axis Z during the time. The scale is negative, that means that when the graphic rise, the transpalet is going down his cm position, and the opposite when the graphic descent.
Then we can see, at first, when the transpalet touch the surface, later it`s more or less stablish, the bad stabilization is because of the high velocity of the transpalet, next start to rise the bump, and we can appreciate three sections. The first one is the first rise movement, later of the step is the moment when the front wheel loss the contact, and later of the second step is when the back wheel loss the contact. Then both get the contact again until stabilizing their position.
Now i repeat the process for the other simulations:
Simulation 2 – 45º wheels
Here we have the same situation, but now we can see a bigger time without contact, this is produce because of the higher cm and the same velocity, means that the potential energy is higher, and it’s during more time on air before touch the ground again.
The same for the back wheel, the time on air is bigger.
And now we can see almost the same situation for the cm position point. i have to differenciate two mains situations. The first one is that now we dont have the steps during the rise process, that means the connection between the wheels and the surface is more stable. And the second and main difference is during the last part, before to achive again a stable position over the ground, there are a elevation os the wheels, we can start to see the influences of a higher cm point.
Behaviour of this last part, the transpalet trying to find again his stability with the ground.
And now, in the last simulation, with the highest position possible, we are going to see this situation maximized.
Simulation 3 – 90º wheels
At first, we can see that now the time of lossing contact for the front and the back wheels is even bigger, and we can see a big step later, this is the moment when the wheels touch again with the ground, and like the velocity function is linear, when they touch, their velocity had been increased.
As i said, the same situation for the back wheels.
As is said before, the behaviour is repeated. We have the first part of horizontal movement over the ground, with some interferences caused for the velocity of the simulation. Later we have the rise of the bump and the descent. Now during the rise we can see a slope change, the higher slope start when the front wheel loss the contact.
Later the wheels touch the ground and back to their normal position, but the potential forces continue yet inside of the transpalet and now is more difficult reach the stabilization. In fact, when my simulation ends, the 3D situation is that:
And here we can see the torque produce during this last part:
We can see a big negative value, that try to compensate the positive produce immediately before.
Now i can say that this problem is real, in spite of all the simplifications and design reductions, i got a body enough realistic to simulate this situation and can see this real problem. Because of the design, i use higher velocities to can see better the results, but if we would create a complete model with every charge and simulate later, we could see the same result because the loss of stability depend on the position of center of mass. The only possible situation to control this behaviour is moving as slow as possible, to can see the move and prevent the dissaster aplying new forces to compensate the situation.
For that reason, big companies or people that work moving big charges with this type of machines, they use a electric one, that produce higher forces to can move and control this types os problems. Because the manuals are so difficult to control if you do not have a perfect horizontal surface.
And as personal conclusion, i can say that doing that project i really learned to use Adams view software. I could choose my project and this made me be more motivated even when all the problems were coming. Now i’m happy to find the expect results and can understand it.
– Class slides and pdfs
– Adams View Help