Gentlemen,
Good comments, but I was talking in general lift installation language. I understand the difference between the terms level, plumb and perpendicular, but field language has a way of evolving.
The majority of lift installers will use the term level when talking about runway type lifts, runways level, cross tubes level etc., and plumb when talking about two-posts. Perpendicular is rarely used. When plumb is used, it's usually both directions, with a level, and the bubble neatly between the lines, gravity.
Perpendicular columns to a sloped floor would result in un-plumb columns and unlevel pads just as plumb columns on a sloped floor will also result in unlevel pads (relative to left and right pads respectively) – assuming you did not raise the low side column.
Let’s assume our subject floor has a one-inch slope east to west with the high side being on the right column. (My comments are not related to floor sloping from front to back or vice versa. Mainly lateral slopes that have a way of dramatically affecting pad height due to the lateral outreach of the lift arms). We’ll assume the overall width on our subject floor is 12-feet at outside of base plate for the floor plan. That would be less than 1/8” of slope every foot. Could easily be perceived as a normal floor.
We have three options…
• Install the lift directly on the surface and shim only as necessary to plumb the columns. In this case, your lift pads would be one inch off – right to left. Assuming where your arms are positioned on the vehicle and the distance between pads, that one-inch could translate to dramatically sloped loading.
• Install the high side (right) column directly on the surface and elevate the low side (left) column an additional one inch with steel shim plates and Quikrete. Plumb both as required. In this case, even though the floor is sloped one inch, the columns would be installed plumb and the lift pads would be equal. This may be considered the “correct” way to do it, however you now would have 25% less embedment on the anchor bolts assuming longer ones were not purchased, and you would have different pad heights relative to floor surface with respect to vehicles sitting on the floor. You may squeak a lift pad under the right side of the car, but find out the left side pad is too high for your ground hugger.
• Third way – as I attempted to describe in previous post with final outcome being level pads and parallel columns.
I reviewed some engineering calculations assuming a 12,000-pound capacity two-post floor-plate model (with no top beam) – a kind of worst case. A ½” shim on the outside edge (or inside edge) of the base plate would cause the base plate to be at a 1.35-degree angle with respect to the floor. Lifts with smaller base plates more – maybe 1.75 degrees. A shim that thick would adjust the pad height position considerably - at least 2” of vertical movement just on the one column side. That is why in most cases, shims no thicker than 3/16” are all that is ever required to adjust pad height.
Assume a 12K two-post floor-plate model has been shimmed ½” on just one edge of the base plate only and is angled about 1.35 degrees based on the dimension of the typically sized base plate. Assume it has an arm that measures 45.1” to the center of the lift pad from the main column centerline when loaded. NOTE: The arm may be longer, but when positioned as it would be during vehicle lifting, the pad would be approximately 45.1” from the main column centerline = moment. The column will also be tilted 1.35 degrees with respect to vertical. When fully raised, the lift head is 75.6” above the floor and the arms move horizontally 1.78" from their original position. The moment arm has increased from 45.1” to 46.9” which is an increase of 4.0%. Due to the moment arm increasing 4%, the column bending moment arm will also increase by 4 %. Therefore, the moment applied to the column and transmitted to the base plate will increase 4%. A 4% additional moment load would increase the anchor bolt tension load from 2772 lbs to 2882 lbs. A single Wej-It 3/4” anchor bolt embedded 5” deep in 3000PSI concrete would have a rated tension capacity of 12,000 pounds or more. Two post lifts with a top beam would have less of a bending moment penalty due to shimming. Main lift structural components would be virtually unaffected by this slight increase in moment loading.
Bottom line, raising vehicles not perfectly centered between your columns will add more stress on the column, base plates, and anchors, than shimming out of plumb a degree or two. Either case, the stresses are very minimal. (At least on ETL certified lifts or those designed with similar criteria.) Two-post lifts rarely need shims greater than 1/4" thick, but you would be surprised what affect even the slightest shimming has on pad height adjustment.
Jeff
BendPak
