Lub luag haujlwm tseem ceeb ntawm lub cev muaj zog hydraulic yog hloov lub zog hydraulic rau hauv lub zog siv tshuab. Cov qauv tsim sab hauv yog txhua yam kom tau raws li qhov xav tau ntawm lub zog hloov dua siab tshiab. Cov khoom tseem ceeb yog raws li nram no.