This is where the grease nipple goes in
	Normally it would look like this, but unfortunately the coupler collides with the edge of the ball bearing holder, so that it can't be fully attached and grease leaks out when pressure is applied.
	I took a 6mm washer, filed off a bit on one side...
	...and put it under the grease nipple. That produced enough space for the coupler to fit tightly.
	Press in grease until you see it coming out of the thread, then manually move the arm up and down to spread the grease inside the ball bearing. Repeat this if necessary.

The set of grease nipples I got contained only 5 of the small, straight ones, so to lubricate the last ball bearing I used one of the 45 degree nipples, which I removed afterwards. The nipples don't need to be attached during normal operation,

[1] https://www.amazon.de/dp/B07TLBXC3K
[2] https://www.amazon.de/dp/B07S7V5TGN
[3] https://www.amazon.de/dp/B0B5KXJPVQ
[4] https://www.amazon.de/dp/B00295JXKC