Ib hom nrov steel sawb caij nplooj ntoos hlav tsim-Variable force springs.Cov springs no muaj nyob rau hauv POP teb, tshwj xeeb tshaj yog siv nyob rau hauv lub kiv cua thiab upward txee zaub.Lub zog sib txawv tuaj yeem muab cov zog sib txawv kom haum rau lub zog xav tau, lawv tuaj yeem thawb rau ...
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