Rashi, Targum Yonasan: Elchanan is David. Radak - I do not know why he is called Elchanan.

Rashi (from Rus Rabah 2:2), Targum Yonasan: His family used to weave the Paroches for the Mikdash, which is called Ya'ar. Radak - we find "Ya'ar ha'Levanon ¹ " and "bi'Sdei Ya'ar" (Tehilim 132:6) refer to the Mikdash.

Malbim: He was from the family of Betzalel, the head of the weavers [for the Mishkan] and "Choshvei Machashavos" (Shemos 35:35). Also David was from this family, therefore he is called Efrati (Betzalel's great grandmother was Efrat (Miryam) - Divrei ha'Yamim I, 2:19. Also David is from her - Sotah 11b.)

Radak #1: Ya'ir is the same as Ya'ar. Here it Es Galyus, i.e. the one who was with him, i.e. his brother. Elchanan killed him in Nov, and David killed Galyus in Efes Damim (Shmuel I, 17:1). Elchanan was from Beis Lechem, therefore he is called Beis ha'Lachmi. In Divrei ha'Yamim it says Es Lachmi; this is like mi'Lachmi, i.e. from Lachmi's sons, i.e. from the Giborim attributed to Beis Lechem. ¹

Radak #2: Here, Beis Lechem is like Im Beis Lechem (with David of Beis Lechem); he helped Elchanan ² . Also there, Es Lachmi means with David, who is Lachmi (from Beis Lechem); he helped [Elchanan], and they killed [Galyus' brother].

Malbim: Lachmi was a Gibor like his brother Galyus. Because David, from Beis Lechem, killed him, people who make parables ³ said 'the Lachmi killed Galyus', to recall the similar Gevurah of David. In Divrei ha'Yamim, it explains what truly happened (Elchanan killed Lachmi).

Rashi, Targum Yonasan: It is the beam on which weavers arrange the warp (lengthwise) threads on a loom. A [descendant of] a weaver should stand against [one whose spear stick is like the beam of ] a weaver.