Material Lift Application
A manufacturer of steel frames used in the commercial dairy industry is building a material lift which contains a stack of prefabricated frames. The material lift will index up as each frame is removed by an automated grip from the top of the stack. The jack will index up 1" in 2 seconds every 30 seconds. After the last frame is removed, the worm screw jack systems will fully retract to the collapsed position in 6 seconds waiting for the next load of frames. Complete cycle time is 10 minutes running 6 hours per day, 5 days per week. The design calls for a four jack arrangement lifting from underneath the lifting stage, driven by a single motor.
Step 1: Determine Specifications
- When fully loaded with frames the total weight is 16,800 lbs
- When fully extended the total load is less than 5,000 lbs
- The load will be in compression
- Total travel: 6 inches
- Desired design life is one year
Step 2: Analysis
Due to frequent cycles and design life, the use of a ball screw jack would be best suited for this application. Linear motion systems with upright translating jacks will allow the jacks to be located under the material lift and not create any obstruction with the loading and unloading of the prefabricated frames.
The jacks will be fully loaded in the retracted position and be unloaded as they extended. Considering the worst case scenario with the full load at the full extended position, the load will be 16,800 lbs lifted by four jacks, and extended to 6" of travel. When considering column strength, the 2.5-BSJ-U jack can be used.
Speed and Horsepower:
The extension travel rate of 1" in 2 seconds is 30"/min. The retraction travel rate of 6" in 6 seconds equates to 60"/min. This would require using the 6:1 gear ratio when using a 1750 rpm AC motor during retraction and 720 rpm during extension. Using the horsepower formulas on page 15 of our Worm Gear Screw Jacks Catalog for the total horsepower need when fully loaded is 3.0 HP with a dual speed 1750/800 AC motor.
|HPper jack =||.0102 in-lb × 4,000 lb × 720 rpm||= .466 hp / jack (during extension)|
|HParrangement =||.466 hp per jack × 4 jacks||= 3.0 total hp (during extension)|
|.85 × (.9)3|
LinkJac Line Shafting:
The longest center to center distance between any jack and gearbox is 36 inches, leaving a shaft length of 29 inches. Using the formula on page 189 based on Torsional Twist, the proper Line Shafting to use would be the LJ-12.
|NSpeed =||.6192 × (||w||)2 × 5.851 × 105 = 4,252 rpm|
|NTwist =||169 in-lb × (||29 in||) = .784o|
Based on the indicated cycle rate the total number of cycles over three years is 9,360 cycles. Considering the worst case scenario with the full load to the full extended position, then unloaded for the retraction, the loaded travel inches is just over 56,160 inches. When considering the life expectancy chart on pages 28-29, the 2.5-BSJ-U jack can be used.
Step 3: Product Selection
Create a Reference Number for the Assembly
From page 31, create reference numbers for the following: four 2.5 ton ball screw jacks, inverted rotating configuration, 6:1 worm gear ration, top plate, 6 inches of travel. One jacks to have a 2 circuit limit switch.
- (3) 2.5-BSJ-U 6:1/SSE-1/SSE-2/FP/6/S
- (1) 2.5-BSJ-U 6:1/2CA-3C/SSE-2/FP/6/S
- (2) GB210S Type E
- (1) GB210S Type D
- (4) LJ-12 29" OAL
- (2) LT-12 14" OAL
- (4) P-2200-185 (½ - ¾)
- (8) P-2200-178 (¾ - ¾)
- 3 HP AC Motor capable of a dual speed 1750/800