TECHNICAL SUPPORT
Published 2026-03-24
Friends who are new toservos often see parameters such as "15 kg", "20 kg" and "25 kg". Their first reaction is, can this thing lift 15 kg? In fact, this understanding is not completely correct. Today we will break it up and talk about what the kilograms on theservomean, so that you will no longer be confused when choosing aservoin the future.
To put it bluntly, this 15 kilograms refers to the torque of the steering gear, and the unit is kg·cm. It means that with a force arm length of 1 cm, the servo can pull a weight of 15 kg. You can think of it like using a wrench to tighten a screw. The longer the wrench, the less effort it takes, and conversely, the shorter the lever arm, the greater the force required. For a 15 kg torque servo, if the moment arm is 1 cm, it can output a pulling force of 15 kg; if the moment arm is 2 cm, the pulling force is halved to only 7.5 kg. So this number represents the torque, not the simple load-bearing capacity.
In practical applications, you have to calculate the length of the moment arm of your project. For example, the distance between the robot's thigh joint and the servo to the center of gravity is 5 cm. The actual load that a 15 kg torque servo can bear is 15 divided by 5, which is only 3 kg. Including the safety margin, it may only be suitable for projects of about 2 kg. When selecting a model, don’t just look at the numbers, you have to calculate it based on your own mechanical structure.
The key to choosing a kilogram servo depends on what your project is for. For a small model aircraft, a 9-gram servo is enough, and the torque is about 1.5 to 2.5 kilograms. If it is a 10cm desktop robot, a 15kg torque servo is basically enough. But if you want to build a 20-centimeter-tall bipedal robot, or if the robotic arm needs to grab things, you may have to start at 25 kilograms. Remember one principle: It is better to have more torque than less torque, but you cannot blindly pursue high torque.
It also depends on the usage scenario of the servo. If it is just used to control the camera gimbal, 5 to 10 kg is more than enough. But if it is used on a racing truck, the steering servo does not weigh more than 20 kilograms, and it will not be able to turn at high speed. It is recommended that you estimate the load first, multiply it by the length of the moment arm, and then multiply it by 2 to 3 times the safety factor. The calculated number is the minimum torque value you need. The servo selected in this way can last a long time.
Let’s teach you how to calculate torque step by step. The first step is to measure the length of the force arm, which is the vertical distance from the steering gear output shaft to the center of gravity of the load, in centimeters. The second step is to weigh the load in kilograms. The third step is to multiply the load weight by the length of the moment arm to get the theoretical torque value. For example, if the load is 2 kg and the moment arm is 3 cm, the theoretical torque is 6 kg·cm. This algorithm is very simple, and junior high school physics knowledge is enough.
But the theoretical value is not equal to the actual demand, and you have to add a safety factor. Because the impact force at the moment of starting is greater than that during smooth operation, coupled with factors such as motion inertia and frictional resistance, the actual required torque is at least twice the theoretical value. Still the same example just now, the theoretical weight is 6 kg, you have to choose at least a servo of 12 kg or more. If used in scenes with large dynamic changes, such as robots running and jumping, the safety factor should be increased to more than 3 times.
Don’t just look at kilograms as one parameter. Voltage has a particularly large impact on torque. For the same steering gear, if it uses 7.4V voltage and 6V voltage, the output torque can differ by 20% to 30%. So when buying, check the rated voltage clearly, and then check how many volts of power your project can provide. In addition, we should also pay attention to metal gears and plastic gears. High-torque servos must use metal gears, otherwise the plastic teeth will easily be scrapped.
Speed is also an important parameter. High-torque servos usually turn slowly because they have to drive heavy loads. If your project requires both strong force and quick response, you will have to look for a high-speed and high-torque servo, which will be much more expensive. There are also the control accuracy of the servo, the operating temperature range, and whether it is waterproof and dustproof. These should all be considered based on your actual use environment. Don’t think that everything will be fine just by buying a 15kg one.
When I get a new servo, how do I know if it really weighs 15 kilograms? The simplest way is to test with a heavy object. Find a 1cm long rudder arm and install it, fix the steering gear horizontally, and then hang a heavy object on the rudder arm to see if it can lift the heavy object. Be careful not to hang it directly with an iron block. It is best to use a mineral water bottle to slowly add water, so that you can know exactly how much weight it can lift. The voltage must be stable at the rated value during testing, otherwise the results will be inaccurate.
A more professional approach is to use a tension gauge or torque tester. Fix the rudder arm on the tension gauge, send a signal to the steering gear to make it rotate, and the value read is the actual output torque. Generally, the manufacturer's nominal 15 kg refers to the stalled torque, which is the maximum output value when the servo cannot rotate. This value cannot be reached in actual work, so during testing it was found that a difference of one or two kilograms is normal. But if the difference is too big, it may be a false mark and you need to change the brand quickly.
There are so many steering gear brands on the market that it’s dizzying, how do you choose a reliable one? First check the reputation, go to model aircraft forums and robot communities, and see what brands old players use. Japanese brands such as Sanwa are indeed of good quality, but they are also expensive and suitable for projects that require extremely high reliability. Domestic brands such as Huisheng, Yinyan, and DS are cost-effective. You can buy a 15kg servo for just a few dozen yuan, which is enough for entry-level use.
Don't be greedy for cheap and buy those no-brand brand-name products. False specifications of parameters are the norm. You obviously bought 15 kilograms, but the actual price may not even be 10 kilograms. What's even more terrible is that the gears of off-brand servos are of poor quality and the circuit protection is incomplete. They will burn out after being used. By the time half of the project is reworked, it will be called a toss. It is recommended to choose brands with regular manufacturers and after-sales services. It is definitely worth spending an extra twenty or thirty yuan to buy peace of mind.
Seeing this, you should have a clear understanding of the meaning of 15 kilograms of steering gear. Next time you choose a servo, will you calculate the load first or just look at the kilograms? Welcome to share your selection experience in the comment area, or talk about the steering gear problems you encountered in the project, let's communicate and solve them together!
Update Time：2026-03-24
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