It's not only the weight, You also have to note the COG.
You can use the free software 'Kuka load' to check the values.
Fill your system data in at the following link:
That will then give you more information regarding what the robot can handle.
Hermann is right in that it is not just weight that is taken into account, you also need to take account of the inertia of the tool. So even though your tool might be within payload it may have high inertias causing the system to be overloaded because of it.
If you can't use Kuka compose then you can use the free Kuka Load software recommended by hermann.
