Reduction unit, arm joint provided with two or three reduction units respectively, and a robot arm

An arm joint comprising:

• a first coupling part connectable in a manner rotatable around a first axis with a first arm part or an external object;
• a second coupling part connectable in a manner rotatable around a second axis or fixedly with a second arm part;
• a third coupling part connected in a manner rotatable around a third axis with the first coupling part, wherein the third axis includes an angle with the first axis in the range of 30-60 degrees, wherein the third coupling part is connected in a manner rotatable around a fourth axis with the second coupling part, wherein the fourth axis includes an angle with the second axis in the range of 30-60 degrees, and wherein the third and the fourth axis mutually include an angle in the range of 60-120 degrees;
• and
• a drive assembly configured to energize the rotation of the first coupling part around the third axis with respect to the third coupling part, the drive assembly being configured to energize the rotation of the second coupling part around the fourth axis with respect to the third coupling part.