Automatic levelling and locking ladder

Claim 1

In a self levelling base structure for use in combination with a ladder, said ladder comprising a pair of ladder rails, said base structure including a pair of parallel, spaced apart upstanding leg members, means for mounting said leg members for vertical sliding movement relative to said rails, each said leg member being provided with a rack member, a pinion member engaging each said rack member, said pinions being coupled by shaft means, a housing for said shaft means, means for rotatably supporting said shaft means within said housing, the last said means permitting generally vertical relative movement between said shaft means and said housing, first bias means urging said housing upwardly relative to said shaft means, and blocking means engageable with said shaft means to restrict rotary movement thereof when said housing moves downwardly towards said shaft means in opposition to said first bias means, the improvement wherein automatic locking means is provided for restricting upward movement of said housing relative to said shaft means, said locking means comprising a locking member mounted within said housing for movement between a first position permitting relative vertical movement between said housing and said shaft means and a second position interposed between said housing and said shaft means to restrict said relative vertical movement, a second bias means urging said locking member to said second position, latching means associated with said shaft means and said housing permitting movement of said locking member to said second position as said housing moves downwardly relative to said shaft means, and means for returning said locking means to said first position.