We must model the motion of a point on the path of a Belt.
To do this, we design two motions for the Point: its Xaxis and Yaxis motion components.
We give the each motion to a different Slider. We configure the two sliders as 'Piggyback Sliders'.
When we cycle, the Yaxis Slider moves in the Yaxis direction. The Yaxis Slider is being carried by the Xaxis Slider. Thus, the Point also move in the Xaxis direction.
If we design the motions correctly, the Point on the Yaxis Slider has the same motion as a Point on a Belt.
In this Tutorial Step, we add a set of Piggyback Sliders. We call the two Sliders a Slider Set.
We design the two motions for SliderX and SliderY in Tutorial Step 15.2.
Note  From Release 11+, it is easier to use a sketchloop and MotionPath FB. See Also: Tutorial 11.
However, we should learn how we go about the model the piggyback sliders, as it is an important modelling method.
What is a Piggyback? I am not sure if it is only an English term. When someone is off the ground and on the back of a standing person, and their hands are around the standing person's neck (to hold on, not to strangle!) and their legs are on each side of the standing person with their legs held up by the standing person's arms, then the person that is off the ground is being given a 'Piggyback'. The motion of the person on the back is a combination of their own motion and the motion of the person on the ground. Why Piggyback Sliders? We can design a motion for a Point to move along the X direction and a different motion to move along the Y direction, The total motion give the motion of a Point and Plane that moves (translates ) in the XY Plane. The total motion of the Slider that is supported = Motion Slider 1 + Motion Slider 2. Why not Piggyback Rockers? Yes we can have PiggyBack Rockers also  this is exactly like a SCARA robot  see below. However, it is almost impossible to predict exactly what the motion of the Point at the end of the second Rocker will be if we give the motions, in angle [degrees], to the Rockers only. (Note: we can still design a motion for the XY Plane for a SCARA robot in MechDesigner. In fact we use Piggyback Sliders, as a simple trick.) New thoughts from the author since writing the above Piggyback Sliders Configuration. We add a Slider to the BasePart that moves along the Yaxis of the BasePart. (SliderY). Add SliderX to SliderY in the Yaxis direction of the SliderY (in the horizontal direction of the BasePart). A Point in SliderX with coordinates (x,y) has coordinates (X+x, Y+y) in the BasePart. 
We add the Piggyback Sliders as follows:
A Point, called PointXY, is a child to the SliderY. The movement of the Piggyback Sliders and the Point XY, is as follows:

Add the XSlider The XSlider will move along the Xaxis of the MechanismPlane . It is the horizontal motion of the point on the belt. 

To add the Line, drag from LefttoRight. The positive direction of the Line will agree with the positive Xaxis of the BasePart.
Drag from LefttoRight to add the Part. Add the Part adjacent to the Line. The positive direction of the Part agrees with the positive Xaxis of the BasePart.
The positive direction of the XSlider and MotionDimension is to the right. Edit the BaseValue of the MotionDimension 

Edit the XSlider, Add a Vertical Line 

Edit the Length of the CADLine to 100mm  my habit.
Drag from BottomtoTop to add the Line. The positive direction of the Line agrees with the positive Yaxis of the XSlider Part.
The length of the Line is 50. The Origin of the Line is Coincident, not merged, with the Origin of the XSlider.


Add the YSlider The YSlider will move in the Yaxis of the XYPlane. It is the vertical motion component of the point on the belt. 

Drag from BottomtoTop to add the Part. The positive direction of the Part agrees with the positive Yaxis of the XSlider Part.
The positive direction of the YSlider and MotionDimension is to the Top. The BaseValue of the MotionDimension is 100. You can specify the BaseValues . Edit the BaseValues to offset the 'belt' in the horizontal and vertical directions. 