三(san)(san)輥穿(chuan)孔的橫斷面變形(xing)(xing)(xing)過(guo)(guo)程是由圓(yuan)形(xing)(xing)(xing)到圓(yuan)三(san)(san)角形(xing)(xing)(xing)再到圓(yuan)形(xing)(xing)(xing)的過(guo)(guo)程。這種三(san)(san)角形(xing)(xing)(xing)的變形(xing)(xing)(xing)特征，使毛管尾(wei)(wei)部易出現三(san)(san)角形(xing)(xing)(xing)。通常，鋼管外徑與(yu)壁厚之比(bi)D／S大于9時，會出現尾(wei)(wei)三(san)(san)角形(xing)(xing)(xing)，D／S值愈大，尾(wei)(wei)三(san)(san)角形(xing)(xing)(xing)也愈大。尾(wei)(wei)三(san)(san)角的形(xing)(xing)(xing)成，限制了在三(san)(san)輥穿(chuan)孔機上穿(chuan)制薄壁管，這是其缺點之一。

  三輥(gun)(gun)穿(chuan)(chuan)孔(kong)(kong)機軋(ya)輥(gun)(gun)形成(cheng)(cheng)了一個近似封閉(bi)的(de)(de)孔(kong)(kong)型，不需(xu)要(yao)像在(zai)(zai)二輥(gun)(gun)穿(chuan)(chuan)孔(kong)(kong)機上為保持管(guan)坯在(zai)(zai)中(zhong)心線上和限(xian)制金屬的(de)(de)橫向流(liu)動，必須(xu)裝設導(dao)(dao)(dao)板(ban)或導(dao)(dao)(dao)盤。因此采用(yong)三輥(gun)(gun)穿(chuan)(chuan)孔(kong)(kong)機穿(chuan)(chuan)孔(kong)(kong)就(jiu)消(xiao)除(chu)了現有二輥(gun)(gun)穿(chuan)(chuan)孔(kong)(kong)機存在(zai)(zai)的(de)(de)缺點。不僅消(xiao)除(chu)了不利導(dao)(dao)(dao)板(ban)或導(dao)(dao)(dao)盤所造成(cheng)(cheng)的(de)(de)金屬應力狀態(tai)，也(ye)消(xiao)除(chu)了過大能(neng)量消(xiao)耗的(de)(de)導(dao)(dao)(dao)板(ban)和導(dao)(dao)(dao)盤，消(xiao)除(chu)了管(guan)坯與它們(men)之間的(de)(de)摩(mo)擦。降低了設備能(neng)量消(xiao)耗，同(tong)時(shi)也(ye)消(xiao)除(chu)了導(dao)(dao)(dao)板(ban)、導(dao)(dao)(dao)盤的(de)(de)工具成(cheng)(cheng)本(ben)，提高了經濟(ji)性。