Loading

Report as inappropriate

Are you sure that your belts aren't tight enough? Which ultimaker do you have and how old is it? On a newer ultimaker original I have, they are not necessary, neither on ultimaker 2.
If it's old and they need more tension, then just loosen the set screw in one pully with a hex screwdriver provided with your ultimaker, put the tensioner on and retighten the set screw.