Siman 41 . The Law of One Who Bears a Burden - How He Should Behave with Tfilin

41.1: (1) One who bears a burden (2) on his head (3) removes the head-tfilin until he removes the burden, and even a "mitpaha" [a light article of clothing, such as a scarf or even handkerchief] cannot be put on his head when he has tfillin on. However, something which it is normal practice to put on his head, such as a hat or "mitznefet" [another type of head-covering] (4) is permitted. {RAMAH: And even so, if it is a burden weighing (5) 4 kabs and the tfillin are (6) pushed aside, then he must remove them (Beit Yosef)}

MB 1: One who bears a burden - This law speaks of one who wears tfillin all day, and not merely during the recital of Sh'ma and prayer. And see below in Siman 97 Para. 5.

MB 2: On his head - And if he bears a burden on his arm at the place of the tfillin, which are probably covered and there is no disgrace, then he does not have to remove the hand-tfillin unless the burden is four kabs, as then the tfillin are probably being crushed by this. (Pri Mgadim, see more there)

MB 3: Removes the head-tfillin - And if he can move the burden to the sides, not on the place of the tfillin, it is implied from the Pri Mgadim that it is permitted. Nevertheless, if the burden is 4 kabs, it appears that we should be stringent even in such cases. See the Beit Yosef. And to take out garbage on his head, even a little and even to the sides, is forbidden, as it is in any case a disgrace for the tfillin on his head, unless he removes the head-tfillin - then it is permitted even though he bears the hand-tfillin. And if he should wish to take out the garbage on his arm, it is possible that even if the tfillin are covered by his clothes, there is abasement to the tfillin which are on him (Pri Mgadim).

MB 4: Permitted - Even though it rests on the tfillin, there is no abasement to the tfillin by this since it is the manner of clothing. But if the hat is large he should be careful that the weight of the hat not remove the tfillin from their proper place.

MB 5: 4 Kabs - This is 25 litraot in which they weigh silver in Prague. (Lehem Hamudot) [I don't understand the exact meaning of this, but it defines the amount of 4 kabs - SB]

MB 6: Pushed - This does not mean when the tfillin move from their place completely and are not located according to the law, as in that case it is obvious that it is forbidden even if the burden is light, but rather he means that the head-tfillin are barely in their place due to the burden which rests on them and reduces the space available for them. And the Bait Hadash is lenient in this matter in the case of a hat and mitznefet. (Pri Mgadim).