The design of a multi-axis linear system goes through different steps: from the study of the needs to the sizing of the system and the choice of the type of guide, discover the essential steps that allow our engineers to size a custom linear system. In this article, let’s take the example of a 3-axis palletizer handling boxes of cosmetic products.
As with any project, the first step is to gather accurate information about the customer’s needs.
For the design of a guidance system, the information to consider is :
- the working environment;
- desired accuracy and repeatability;
- the length of the strokes involved;
- the load on board and its center of gravity;
- Exact duty cycles.
Based on this data, a preselection of suitable materials can be made to meet the requirements. The working environment determines the choice of a type of axis, open or protected according to the degree of pollution. In our case, open axles will suffice since the environment is clean. The environment as well as the necessary speeds can also condition the choice of linear guides: ball or roller.
The precision and repeatability will then guide us towards a choice of transmission. If the desired precision is high, it will be appropriate to choose a ball screw drive. Otherwise, a belt drive or a rack and pinion system will be sufficient. As the production of a pallet of cardboard boxes does not require a high degree of precision, a belt drive will be perfectly suitable.
At this stage of the design process, the multi-axis system must be dimensioned, which in our case is structured as follows :
- 2 horizontal axes (X) connected by a synchronization shaft ;
- 1 horizontal axis (Y) mounted transversely on the X carriages;
- 1 vertical axis (Z) fixed on the Y axis for the up/down movements.
- The dimensioning is always done starting from the Z axis and then going up on the Y and X.
The criteria to be considered for the sizing of the Z axis are :
Since the axis is composed of a fixed carriage and a moving profile, it will be necessary to calculate the total mass of the moving profile in order to choose a belt with a maximum safety coefficient. The length of the stroke will determine the rigidity of the axis. In case of a long stroke (more than 1500 mm), it is essential to have a profile with a large section and 2 guide rails to increase the rigidity of the assembly.
Dimensioning of the Y axis: distance between fixing points and stroke
Let’s move on to the Y axis. The elements to appreciate are :
- the distance between the 2 fixing points on the X axis ;
- the stroke on the Z axis.
Rollon transverse axes are rectangular section profiles mounted on a field. The Y axis being self-supporting, its deflection (located in the middle of the axis) must be checked according to the center distance and the load on board. It is relevant to change to a larger section of profile to limit the deflection and have a higher moment of inertia. A greater stability of the Y-axis will be obtained if the stroke of the Z-axis is consequent.
When dimensioning the horizontal axes, the deflection must be checked if the profiles are only attached at their ends. If the distance between the X’s is important and if the drive dynamics are high, it will be necessary to foresee 2 synchronization shafts on each side with a geared motor in the center of the 2 modules to avoid transmission delays.
There are many criteria to take into account in order to build an efficient multi-axis system.