Trailer Ramp Springs - Case Study
Trailer Ramp Spring-Casa Study
We recently worked with a customer who was looking for torsion springs (for his trailer ramp). He had purchased trailer ramp springs from a different company. The other company was not a spring company, rather a trailer ramp company. From what the customer explained, they had different spring ratings and they sold him some that fit, but the springs were failing; they were breaking after just a few times of deflection.
We informed the customer that we could design custom springs to meet his specific trailer ramp needs. He was asked to provide as much information as possible. The following information is the specific things that we required in order to assist him:
- Dead weight of ramp
- Length of ramp from the hinge, out (in feet)
- Distance traveled from the ramp’s upright position to the floor (in degrees of deflection)
- Number of springs that can be used
- Diameter of shaft going inside the spring’s inner diameter
- Space available for the spring’s body
Items 1 to 3 are to determine the spring’s torque (force) and elasticity. In the case of this customer, his ramp measured 5 feet from the hinge out, and weighed about 250 pounds. From the upright position to the ground, the ramp would travel/deflect about 115º (this number is an estimate for it depends on the terrain). With these values, our design engineer was able to calculate the required spring rate of inch-pounds per degree of deflection, using the formula shown below.
- Rate = Weight * Length * 2 ÷ Travel
- k = 2WL ÷ x
- k = 2(250 * 5) ÷ 115
- k = 2,500 ÷ 115
- k = 21.739 in-lbs/deg
Then, we take the information required listed in number 4 to find out how many springs we can use to divide that torque into. Because when springs work in parallel, the force/torque is divided into the number of springs being used. In this case, we used 3 springs which meant each spring would need to have a spring rate of 7.246. The next step is to use the information listed as number 4 and 5, and begin working on a spring design that will fit around the designated space provided for the spring.
This customer had plenty of space but the springs couldn’t be more than 8” in body length, and the inner diameter couldn’t be any smaller than 1.125”. The reason as to why we make the inner diameter larger than the shaft is because, as a spring torques, the inner diameter shrinks, so we had to ensure that the inner diameter after deflection would still fit. To work up a design, we use spring creator. The initial measurements were a 0.375” wire with a 2” OD (giving us an ID of 1.25”), 10 active coils, 4” typical leg lengths, and the material type was Chrome Silicon ASTM A401. This gave us a spring rate of 9.39 in-lbs/deg. and max safe travel of 83º. Next, the outer diameter and number of coils was increased for more elasticity but that made the spring rate weaker, so it was necessary to increased the wire size to the next size (0.394”). The final design was the following:
- Wire diameter:0.394”
- Outer diameter:2.225”
- Inner diameter:1.437”
- Active coils:14
- Leg Lengths:4”
- Material type:Chrome Silicon ASTM A401
- Spring rate:7.254 in-lbs/deg
- Maximum Safe Travel:126º
- Torque at 115º of travel:834.183 in-lbs
- Inner diameter at 115º of travel:1.405”
- Body length:5.910”
The spring would fit well in the space available and work perfectly as a lift-assist. Not only did Spring Creator assist in providing a design and full spring analysis, it does much more. Spring Creator also provides an Instant Spring Quote for our customer, so no need to wait hours or days for pricing. Getting these springs made was more expensive than what the customer had expected, however it was well worth it.
Since it was a specialty spring, custom made to assist him and his wife when lifting the ramp, the time and trouble he would be saving made this specialty spring a great deal and he was able to receive special pricing for 4 pieces so he could also have a spare. Overall, the customer not only saved time and money in finding the getting the perfect spring to lift the ramp but was also able to avoid the trouble of trying to find something that may or may not work elsewhere and that may not have last more than a few days and having to waste even more time and effort. Therefore, the customer was very satisfied with the overall design and order of springs he received.