Quick background, I have hauled all manner of equipment as a profession truck driver. I have had to take special classes and get special certification to haul certain things in order to prove that I know how to secure a load.
The real trick with securing something on wheels is triangulation. What I mean by that is rather than simply throwing a strap over and cranking it down till it is tight like you can with hauling lumber or pipe you must create triangles with your straps that squeeze the load to that it can't roll. Car haulers have it easy because they can simply throw a web over the tires or chains over the axle so that it cant roll, thus loosening the straps. If you have anchor points on your trailer right below the tires of your splitter you could do the same thing, but the chances of that are not good. So what you need to do us strap around the axle pulling it forward and to the tongue pulling it backward. This creates at triangle because if the load tries to move forward the tongue straps pull it back and down into the floor of the trailer. Same thing happens if the splitter tries to roll back, the axle straps pull it forward and down so that it can't go anywhere. In order to prevent lateral movement (side to side) use 2 straps on the tongue pulling back and to opposite sides. This will triangulate the lateral movement as well. Using this method you will need four straps but unless a strap breaks or an anchor point snaps your splitter will never move.
